Chapter XVIII

ON THE ADVANTAGES AND DISADVANTAGES OF CHANGED CONDITIONS OF LIFE: STERILITY FROM VARIOUS CAUSES.

ON THE GOOD DERIVED FROM SLIGHT CHANGES IN THE CONDITIONS OF
LIFE--STERILITY FROM CHANGED CONDITIONS, IN ANIMALS, IN THEIR NATIVE
COUNTRY AND IN MENAGERIES--MAMMALS, BIRDS, AND INSECTS--LOSS OF
SECONDARY SEXUAL CHARACTERS AND OF INSTINCTS--CAUSES OF
STERILITY--STERILITY OF DOMESTICATED ANIMALS FROM CHANGED
CONDITIONS--SEXUAL INCOMPATIBILITY OF INDIVIDUAL ANIMALS--STERILITY OF
PLANTS FROM CHANGED CONDITIONS OF LIFE--CONTABESCENCE OF THE
ANTHERS--MONSTROSITIES AS A CAUSE OF STERILITY--DOUBLE
FLOWERS--SEEDLESS FRUIT--STERILITY FROM THE EXCESSIVE DEVELOPMENT OF
THE ORGANS OF VEGETATION--FROM LONG-CONTINUED PROPAGATION BY
BUDS--INCIPIENT STERILITY THE PRIMARY CAUSE OF DOUBLE FLOWERS AND
SEEDLESS FRUIT.

_On the Good derived from slight Changes in the Conditions of Life._--In considering whether any facts were known which might throw light on the conclusion arrived at in the last chapter, namely, that benefits ensue from crossing, and that it is a law of nature that all organic beings should occasionally cross, it appeared to me probable that the good derived from slight changes in the conditions of life, from being an analogous phenomenon, might serve this purpose. No two individuals, and still less no two varieties, are absolutely alike in constitution and structure; and when the germ of one is fertilised by the male element of another, we may believe that it is acted on in a somewhat similar manner as an individual when exposed to slightly changed conditions. Now, every one must have observed the remarkable influence on convalescents of a change of residence, and no medical man doubts the truth of this fact. Small farmers who hold but little land are convinced that their cattle derive great benefit from a change of pasture. In the case of plants, the evidence is strong that a great advantage is derived from exchanging seeds, tubers, bulbs, and cuttings from one soil or place to another as different as possible. {146}

The belief that plants are thus benefited, whether or not well founded,
has been firmly maintained from the time of Columella, who wrote
shortly after the Christian era, to the present day; and it now
prevails in England, France, and Germany.[323] A sagacious observer,
Bradley, writing in 1724,[324] says, "When we once become Masters of a
good Sort of Seed, we should at least put it into Two or Three Hands,
where the Soils and Situations are as different as possible; and every
Year the Parties should change with one another; by which Means, I find
the Goodness of the Seed will be maintained for several Years. For Want
of this Use many Farmers have failed in their Crops and been great
Losers." He then gives his own practical experience on this head. A
modern writer[325] asserts, "Nothing can be more clearly established in
agriculture than that the continual growth of any one variety in the
same district makes it liable to deterioration either in quality or
quantity." Another writer states that he sowed close together in the
same field two lots of wheat-seed, the product of the same original
stock, one of which had been grown on the same land, and the other at a
distance, and the difference in favour of the crop from the latter seed
was remarkable. A gentleman in Surrey who has long made it his business
to raise wheat to sell for seed, and who has constantly realised in the
market higher prices than others, assures me that he finds it
indispensable continually to change his seed; and that for this purpose
he keeps two farms differing much in soil and elevation.

With respect to the tubers of the potato, I find that at the present
day the practice of exchanging sets is almost everywhere followed. The
great growers of potatoes in Lancashire formerly used to get tubers
from Scotland, but they found that "a change from the moss-lands, and
_vice versâ_, was generally sufficient." In former times in France the
crop of potatoes in the Vosges had become reduced in the course of
fifty or sixty years in the proportion from 120-150 to 30-40 bushels;
and the famous Oberlin attributed the surprising good which he effected
in large part to changing the sets.[326]

A well-known practical gardener, Mr. Robson[327] positively states that
he has himself witnessed decided advantage from obtaining bulbs of the
onion, tubers of the potato, and various seeds, all of the same kind,
from different soils and distant parts of England. He further states
that with {147} plants propagated by cuttings, as with the Pelargonium,
and especially the Dahlia, manifest advantage is derived from getting
plans of the same variety, which have been cultivated in another place;
or, "where the extent of the place allows, to take cuttings from one
description of soil to plant on another, so as to afford the change
that seems so necessary to the well-being of the plants." He maintains
that after a time an exchange of this nature is "forced on the grower,
whether he be prepared for it or not." Similar remarks have been made
by another excellent gardener, Mr. Fish, namely, that cuttings of the
same variety of Calceolaria, which he obtained from a neighbour,
"showed much greater vigour than some of his own that were treated in
exactly the same manner," and he attributed this solely to his own
plants having become "to a certain extent worn out or tired of their
quarters." Something of this kind apparently occurs in grafting and
budding fruit-trees; for, according to Mr. Abbey, grafts or buds
generally take on a distinct variety or even species, or on a stock
previously grafted, with greater facility than on stocks raised from
seeds of the variety which is to be grafted; and he believes this
cannot be altogether explained by the stocks in question being better
adapted to the soil and climate of the place. It should, however, be
added, that varieties grafted or budded on very distinct kinds, though
they may take more readily and grow at first more vigorously than when
grafted on closely allied stocks, afterwards often become unhealthy.

I have studied M. Tessier's careful and elaborate experiments,[328]
made to disprove the common belief that good is derived from a change
of seed; and he certainly shows that the same seed may with care be
cultivated on the same farm (it is not stated whether on exactly the
same soil) for ten consecutive years without loss. Another excellent
observer, Colonel Le Couteur,[329] has come to the same conclusion; but
then he expressly adds, if the same seed be used, "that which is grown
on land manured from the mixen one year becomes seed for land prepared
with lime, and that again becomes seed for land dressed with ashes,
then for land dressed with mixed manure, and so on." But this in effect
is a systematic exchange of seed, within the limits of the same farm.

On the whole the belief, which has long been held by many skilful cultivators, that good follows from exchanging seed, tubers, &c., seems to be fairly well founded. Considering the small size of most seeds, it seems hardly credible that the advantage thus derived can be due to the seeds obtaining in one soil some chemical element deficient in the other soil. As plants after once germinating naturally become fixed to the same spot, it might have been anticipated that they would show the good effects of a change more plainly than animals, which continually wander about; and this apparently is the {148} case. Life depending on, or consisting in, an incessant play of the most complex forces, it would appear that their action is in some way stimulated by slight changes in the circumstances to which each organism is exposed. All forces throughout nature, as Mr. Herbert Spencer[330] remarks, tend towards an equilibrium, and for the life of each being it is necessary that this tendency should be checked. If these views and the foregoing facts can be trusted, they probably throw light, on the one hand, on the good effects of crossing the breed, for the germ will be thus slightly modified or acted on by new forces; and on the other hand, on the evil effects of close interbreeding prolonged during many generations, during which the germ will be acted on by a male having almost identically the same constitution.

_Sterility from changed Conditions of Life._

I will now attempt to show that animals and plants, when removed from their natural conditions, are often rendered in some degree infertile or completely barren; and this occurs even when the conditions have not been greatly changed. This conclusion is not necessarily opposed to that at which we have just arrived, namely, that lesser changes of other kinds are advantageous to organic beings. Our present subject is of some importance, from having an intimate connexion with the causes of variability. Indirectly it perhaps bears on the sterility of species when crossed: for as, on the one hand, slight changes in the conditions of life are favourable to plants and animals, and the crossing of varieties adds to the size, vigour, and fertility of their offspring; so, on the other hand, certain other changes in the conditions of life cause sterility; and as this likewise ensues from crossing much-modified forms or species, we have a parallel and double series of facts, which apparently stand in close relation to each other.

It is notorious that many animals, though perfectly tamed, {149} refuse to breed in captivity. Isidore Geoffroy St. Hilaire[331] consequently has drawn a broad distinction between tamed animals which will not breed under captivity, and truly domesticated animals which breed freely--generally more freely, as shown in the sixteenth chapter, than in a state of nature. It is possible and generally easy to tame most animals; but experience has shown that it is difficult to get them to breed regularly, or even at all. I shall discuss this subject in detail; but will give only those cases which seem most illustrative. My materials are derived from notices scattered through various works, and especially from a Report, drawn up for me by the kindness of the officers of the Zoological Society of London, which has especial value, as it records all the cases, during nine years from 1838-46, in which the animals were seen to couple but produced no offspring, as well as the cases in which they never, as far as known, coupled. This MS. Report I have corrected by the annual Reports subsequently published. Many facts are given on the breeding of the animals in that magnificent work, 'Gleanings from the Menageries of Knowsley Hall,' by Dr. Gray. I made, also, particular inquiries from the experienced keeper of the birds in the old Surrey Zoological Gardens. I should premise that a slight change in the treatment of animals sometimes makes a great difference in their fertility; and it is probable that the results observed in different menageries would differ. Indeed some animals in our Zoological Gardens have become more productive since the year 1846. It is, also, manifest from F. Cuvier's account of the Jardin des Plantes,[332] that the animals formerly bred much less freely there than with us; for instance, in the Duck tribe, which is highly prolific, only one species had at that period produced young.

The most remarkable cases, however, are afforded by animals kept in
their native country, which, though perfectly tamed, quite healthy, and
allowed some freedom, are absolutely incapable of breeding.
Rengger,[333] who in Paraguay particularly attended to this subject,
specifies six quadrupeds in this condition; and he mentions two or
three others which most rarely {150} breed. Mr. Bates, in his admirable
work on the Amazons, strongly insists on similar cases;[334] and he
remarks, that the fact of thoroughly tamed native mammals and birds not
breeding when kept by the Indians, cannot be wholly accounted for by
their negligence or indifference, for the turkey is valued by them, and
the fowl has been adopted by the remotest tribes. In almost every part
of the world--for instance, in the interior of Africa, and in several
of the Polynesian islands--the natives are extremely fond of taming the
indigenous quadrupeds and birds; but they rarely or never succeed in
getting them to breed.

The most notorious case of an animal not breeding in captivity is that
of the elephant. Elephants are kept in large numbers in their native
Indian home, live to old age, and are vigorous enough for the severest
labour; yet, with one or two exceptions, they have never been known
even to couple, though both males and females have their proper
periodical seasons. If, however, we proceed a little eastward to Ava,
we hear from Mr. Crawfurd[335] that their "breeding in the domestic
state, or at least in the half-domestic state in which the female
elephants are generally kept, is of every-day occurrence;" and Mr.
Crawfurd informs me that he believes that the difference must be
attributed solely to the females being allowed to roam the forests with
some degree of freedom. The captive rhinoceros, on the other hand,
seems from Bishop Heber's account[336] to breed in India far more
readily than the elephant. Four wild species of the horse genus have
bred in Europe, though here exposed to a great change in their natural
habits of life; but the species have generally been crossed one with
another. Most of the members of the pig family breed readily in our
menageries: even the Red River hog (_Potamochoerus penicillatus_), from
the sweltering plains of West Africa, has bred twice in the Zoological
Gardens. Here also the Peccary (_Dicotyles torquatus_) has bred several
times; but another species, the _D. labiatus_, though rendered so tame
as to be half-domesticated, breeds so rarely in its native country of
Paraguay, that according to Rengger[337] the fact requires
confirmation. Mr. Bates remarks that the tapir, though often kept tame
in Amazonia by the Indians, never breeds.

Ruminants generally breed quite freely in England, though brought from
widely different climates, as may be seen in the Annual Reports of the
Zoological Gardens, and in the Gleanings from Lord Derby's menagerie.

The Carnivora, with the exception of the Plantigrade division,
generally breed (though with capricious exceptions) almost as freely as
ruminants. Many species of Felidæ have bred in various menageries,
although imported from various climates and closely confined. Mr.
Bartlett, the present superintendent of the Zoological Gardens,[338]
remarks that the lion appears to breed more frequently and to bring
forth more young at a birth than any other species of the family. He
adds that the tiger has rarely bred; {151} "but there are several
well-authenticated instances of the female tiger breeding with the
lion." Strange as the fact may appear, many animals under confinement
unite with distinct species and produce hybrids quite as freely as, or
even more freely than, with their own species. On inquiring from Dr.
Falconer and others, it appears that the tiger when confined in India
does not breed, though it has been known to couple. The cheetah (_Felis
jubata_) has never been known by Mr. Bartlett to breed in England, but
it has bred at Frankfort; nor does it breed in India, where it is kept
in large numbers for hunting; but no pains would be taken to make them
breed, as only those animals which have hunted for themselves in a
state of nature are serviceable and worth training.[339] According to
Rengger, two species of wild cats in Paraguay, though thoroughly tamed,
have never bred. Although so many of the Felidæ breed readily in the
Zoological Gardens, yet conception by no means always follows union: in
the nine-year Report, various species are specified which were observed
to couple seventy-three times, and no doubt this must have passed many
times unnoticed; yet from the seventy-three unions only fifteen births
ensued. The Carnivora in the Zoological Gardens were formerly less
freely exposed to the air and cold than at present, and this change of
treatment, as I was assured by the former superintendent, Mr. Miller,
greatly increased their fertility. Mr. Bartlett, and there cannot be a
more capable judge, says, "it is remarkable that lions breed more
freely in travelling collections than in the Zoological Gardens;
probably the constant excitement and irritation produced by moving from
place to place, or change of air, may have considerable influence in
the matter."

Many members of the Dog family breed readily when confined. The Dhole
is one of the most untameable animals in India, yet a pair kept there
by Dr. Falconer produced young. Foxes, on the other hand, rarely breed,
and I have never heard of such an occurrence with the European fox: the
silver fox of North America (_Canis argentatus_), however, has bred
several times in the Zoological Gardens. Even the otter has bred there.
Every one knows how readily the semi-domesticated ferret breeds, though
shut up in miserably small cages; but other species of Viverra and
Paradoxurus absolutely refuse to breed in the Zoological Gardens. The
Genetta has bred both here and in the Jardin des Plantes, and produced
hybrids. The _Herpestes fasciatus_ has likewise bred; but I was
formerly assured that the _H. griseus_, though many were kept in the
Gardens, never bred.

The Plantigrade Carnivora breed under confinement much less freely,
without our being able to assign any reason, than other members of the
group. In the nine-year Report it is stated that the bears had been
seen in the Zoological Gardens to couple freely, but previously to 1848
had most rarely conceived. In the Reports published since this date
three species have produced young (hybrids in one case), and, wonderful
to relate, the white Polar bear has produced young. The badger (_Meles
taxus_) has bred several times in the Gardens; but I have not heard of
this {152} occurring elsewhere in England, and the event must be very
rare, for an instance in Germany has been thought worth recording.[340]
In Paraguay the native Nasua, though kept in pairs during many years
and perfectly tamed, has never been known, according to Rengger, to
breed or show any sexual passion; nor, as I hear from Mr. Bates, does
this animal, or the Cercoleptes, breed in the region of the Amazons.
Two other plantigrade genera, Procyon and Gulo, though often kept tame
in Paraguay, never breed there. In the Zoological Gardens species of
Nasua and Procyon have been seen to couple; but they did not produce
young.

As domesticated rabbits, guinea-pigs, and white mice breed so
abundantly when closely confined under various climates, it might have
been thought that most other members of the Rodent order would have
bred in captivity, but this is not the case. It deserves notice, as
showing how the capacity to breed sometimes goes by affinity, that the
one native rodent of Paraguay, which there breeds _freely_ and has
yielded successive generations, is the _Cavia aperea_; and this animal
is so closely allied to the guinea-pig, that it has been erroneously
thought to be the parent-form.[341] In the Zoological Gardens, some
rodents have coupled, but have never produced young; some have neither
coupled nor bred; but a few have bred, as the porcupine more than once,
the Barbary mouse, lemming, chinchilla, and the agouti (_Dasyprocta
aguti_), several times. This latter animal has also produced young in
Paraguay, though they were born dead and ill-formed; but in Amazonia,
according to Mr. Bates, it never breeds, though often kept tame about
the houses. Nor does the paca (_Coelogenys paca_) breed there. The
common hare when confined has, I believe, never bred in Europe;[342]
though, according to a recent statement, it has crossed with the
rabbit. I have never heard of the dormouse breeding in confinement. But
squirrels offer a more curious case: with one exception, no species has
ever bred in the Zoological Gardens, yet as many as fourteen
individuals of _S. palmarum_ were kept together during several years.
The _S. cinerea_ has been seen to couple, but it did not produce young;
nor has this species, when rendered extremely tame in its native
country, North America, been ever known to breed.[343] At Lord Derby's
menagerie squirrels of many kinds were kept in numbers, but Mr.
Thompson, the superintendent, told me that none had ever bred there, or
elsewhere as far as he knew. I have never heard of the English squirrel
breeding in confinement. But the species which has bred more than once
in the Zoological Gardens is the one which perhaps might have been
least expected, namely, the flying squirrel (_Sciuropterus volucella_):
it has, also, bred several times {153} near Birmingham; but the female
never produced more than two young at a birth, whereas in its native
American home she bears from three to six young.[344]

Monkeys, in the nine-year Report from the Zoological Gardens, are
stated to unite most freely, but during this period, though many
individuals were kept, there were only seven births. I have heard of
one American monkey alone, the Ouistiti, breeding in Europe.[345] A
Macacus, according to Flourens, bred in Paris; and more than one
species of this genus has produced young in London, especially the
_Macacus rhesus_, which everywhere shows a special capacity to breed
under confinement. Hybrids have been produced both in Paris and London
from this same genus. The Arabian baboon, or _Cynocephalus
hamadryas_,[346] and a Cercopithecus have bred in the Zoological
Gardens, and the latter species at the Duke of Northumberland's.
Several members of the family of Lemurs have produced hybrids in the
Zoological Gardens. It is much more remarkable that monkeys very rarely
breed when confined in their native country; thus the Cay (_Cebus
azaræ_) is frequently and completely tamed in Paraguay, but
Rengger[347] says that it breeds so rarely, that he never saw more than
two females which had produced young. A similar observation has been
made with respect to the monkeys which are frequently tamed by the
aborigines in Brazil.[348] In the region of the Amazons, these animals
are so often kept in a tame state, that Mr. Bates in walking through
the streets of Parà counted thirteen species; but, as he asserts, they
have never been known to breed in captivity.[349]

_Birds._

Birds offer in some respects better evidence than quadrupeds, from
their breeding more rapidly and being kept in greater numbers. We have
seen that carnivorous animals are more fertile under confinement than
most other mammals. The reverse holds good with carnivorous birds. It
is said[350] that as many as eighteen species have been used in Europe
for hawking, and several others in Persia and India;[351] they have
been kept in their native country in the finest condition, and have
been flown during six, eight, or nine years;[352] yet there is no
record of their having ever produced young. As these birds were
formerly caught whilst young, at great expense, being imported from
Iceland, Norway, and Sweden, there can {154} be little doubt that, if
possible, they would have been propagated. In the Jardin des Plantes,
no bird of prey has been known to couple.[353] No hawk, vulture, or owl
has ever produced fertile eggs in the Zoological Gardens, or in the old
Surrey Gardens, with the exception, in the former place on one
occasion, of a condor and a kite (_Milvus niger_). Yet several species,
namely, the _Aquila fusca_, _Haliætus leucocephalus_, _Falco
tinnunculus_, _F. subbuteo_, and _Buteo vulgaris_, have been seen to
couple in the Zoological Gardens. Mr. Morris[354] mentions as a unique
fact that a kestrel (_Falco tinnunculus_) bred in an aviary. The one
kind of owl which has been known to couple in the Zoological Gardens
was the Eagle Owl (_Bubo maximus_); and this species shows a special
inclination to breed in captivity; for a pair at Arundel Castle, kept
more nearly in a state of nature "than ever fell to the lot of an
animal deprived of its liberty,"[355] actually reared their young. Mr.
Gurney has given another instance of this same owl breeding in
confinement; and he records the case of a second species of owl, the
_Strix passerina_, breeding in captivity.[356]

Of the smaller graminivorous birds, many kinds have been kept tame in
their native countries, and have lived long; yet, as the highest
authority on cage-birds[357] remarks, their propagation is "uncommonly
difficult." The canary-bird shows that there is no inherent difficulty
in these birds breeding freely in confinement; and Audubon says[358]
that the _Fringilla_ (_Spiza_) _ciris_ of North America breeds as
perfectly as the canary. The difficulty with the many finches which
have been kept in confinement is all the more remarkable as more than a
dozen species could be named which have yielded hybrids with the
canary; but hardly any of these, with the exception of the siskin
(_Fringilla spinus_), have reproduced their own kind. Even the
bullfinch (_Loxia pyrrhula_) has bred as frequently with the canary,
though belonging to a distinct genus, as with its own species.[359]
With respect to the skylark (_Alauda arvensis_), I have heard of birds
living for seven years in an aviary, which never produced young; and a
great London bird-fancier assured me that he had never known an
instance of their breeding; nevertheless one case has been
recorded.[360] In the nine-year Report from the Zoological Society,
twenty-four incessorial species are enumerated which had not bred, and
of these only four were known to have coupled.

Parrots are singularly long-lived birds; and Humboldt mentions the
curious fact of a parrot in South America, which spoke the language of
{155} an extinct Indian tribe, so that this bird preserved the sole
relic of a lost language. Even in this country there is reason to
believe[361] that parrots have lived to the age of nearly one hundred
years; yet, though many have been kept in Europe, they breed so rarely
that the event has been thought worth recording in the gravest
publications.[362] According to Bechstein[363] the African _Psittacus
erithacus_ breeds oftener than any other species: the _P. macoa_
occasionally lays fertile eggs, but rarely succeeds in hatching them;
this bird, however, has the instinct of incubation sometimes so
strongly developed, that it will hatch the eggs of fowls or pigeons. In
the Zoological Gardens and in the old Surrey Gardens some few species
have coupled, but, with the exception of three species of parrakeets,
none have bred. It is a much more remarkable fact that in Guiana
parrots of two kinds, as I am informed by Sir E. Schomburgk, are often
taken from the nests by the Indians and reared in large numbers; they
are so tame that they fly freely about the houses, and come when called
to be fed, like pigeons; yet he has never heard of a single instance of
their breeding.[364] In Jamaica, a resident naturalist, Mr. R.
Hill,[365] says, "no birds more readily submit to human dependence than
the parrot-tribe, but no instance of a parrot breeding in this tame
life has been known yet." Mr. Hill specifies a number of other native
birds kept tame in the West Indies, which never breed in this state.

The great pigeon family offers a striking contrast with parrots: in the
nine-year Report thirteen species are recorded as having bred, and,
what is more noticeable, only two were seen to couple without any
result. Since the above date every annual Report gives many cases of
various pigeons breeding. The two magnificent crowned pigeons (_Goura
coronata_ and _Victoriæ_) produced hybrids; nevertheless, of the former
species more than a dozen birds were kept, as I am informed by Mr.
Crawfurd, in a park at Penang, under a perfectly well-adapted climate,
but never once bred. The _Columba migratoria_ in its native country,
North America, invariably lays two eggs, but in Lord Derby's menagerie
never more than one. The same fact has been observed with the _C.
leucocephala_.[366]

Gallinaceous birds of many genera likewise show an eminent capacity for
breeding under captivity. This is particularly the case with pheasants;
yet our English species seldom lays more than ten eggs in confinement;
whilst from eighteen to twenty is the usual number in the wild
state.[367] With the Gallinaceæ, as with all other orders, there are
marked and {156} inexplicable exceptions in regard to the fertility of
certain species and genera under confinement. Although many trials have
been made with the common partridge, it has rarely bred, even when
reared in large aviaries; and the hen will never hatch her own
eggs.[368] The American tribe of Guans or Cracidæ are tamed with
remarkable ease, but are very shy breeders in this country;[369] but
with care various species were formerly made to breed rather freely in
Holland.[370] Birds of this tribe are often kept in a perfectly tamed
condition in their native country by the Indians, but they never
breed.[371] It might have been expected that grouse from their habits
of life would not have bred in captivity, more especially as they are
said soon to languish and die.[372] But many cases are recorded of
their breeding: the capercailzie (_Tetrao urogallus_) has bred in the
Zoological Gardens; it breeds without much difficulty when confined in
Norway, and in Russia five successive generations have been reared:
_Tetrao tetrix_ has likewise bred in Norway; _T. Scoticus_ in Ireland;
_T. umbellus_ at Lord Derby's; and _T. cupido_ in North America.

It is scarcely possible to imagine a greater change in habits than that
which the members of the ostrich family must suffer, when cooped up in
small enclosures under a temperate climate, after freely roaming over
desert and tropical plains or entangled forests. Yet almost all the
kinds, even the mooruk (_Casuarius Bennettii_) from New Ireland, has
frequently produced young in the various European menageries. The
African ostrich, though perfectly healthy and living long in the South
of France, never lays more than from twelve to fifteen eggs, though in
its native country it lays from twenty-five to thirty.[373] Here we
have another instance of fertility impaired, but not lost, under
confinement, as with the flying squirrel, the hen-pheasant, and two
species of American pigeons.

Most Waders can be tamed, as the Rev. E. S. Dixon informs me, with
remarkable facility; but several of them are short-lived under
confinement, so that their sterility in this state is not surprising.
The cranes breed more readily than other genera: _Grus montigresia_ has
bred several times in Paris and in the Zoological Gardens, as has _G.
cinerea_ at the latter place, and _G. antigone_ at Calcutta. Of other
members of this great order, _Tetrapteryx paradisea_ has bred at
Knowsley, a Porphyrio in Sicily, and the _Gallinula chloropus_ in the
Zoological Gardens. On the other hand, several {157} birds belonging to
this order will not breed in their native country, Jamaica; and the
Psophia, though often kept by the Indians of Guiana about their houses,
"is seldom or never known to breed."[374]

No birds breed with such complete facility under confinement as the
members of the great Duck family; yet, considering their aquatic and
wandering habits, and the nature of their food, this could not have
been anticipated. Even some time ago above two dozen species had bred
in the Zoological Gardens; and M. Selys-Longchamps has recorded the
production of hybrids from forty-four different members of the family;
and to these Professor Newton has added a few more cases.[375] "There
is not," says Mr. Dixon,[376] "in the wide world, a goose which is not
in the strict sense of the word domesticable;" that is, capable of
breeding under confinement; but this statement is probably too bold.
The capacity to breed sometimes varies in individuals of the same
species; thus Audubon[377] kept for more than eight years some wild
geese (_Anser Canadensis_), but they would not mate; whilst other
individuals of the same species produced young during the second year.
I know of but one instance in the whole family of a species which
absolutely refuses to breed in captivity, namely, the _Dendrocygna
viduata_, although, according to Sir R. Schomburgk,[378] it is easily
tamed, and is frequently kept by the Indians of Guiana. Lastly, with
respect to Gulls, though many have been kept in the Zoological Gardens
and in the old Surrey Gardens, no instance was known before the year
1848 of their coupling or breeding; but since that period the herring
gull (_Larus argentatus_) has bred many times in the Zoological Gardens
and at Knowsley.

There is reason to believe that insects are affected by confinement
like the higher animals. It is well known that the Sphingidæ rarely
breed when thus treated. An entomologist[379] in Paris kept twenty-five
specimens of _Saturnia pyri_, but did not succeed in getting a single
fertile egg. A number of females of _Orthosia munda_ and of _Mamestra
suasa_ reared in confinement were unattractive to the males.[380] Mr.
Newport kept nearly a hundred individuals of two species of Vanessa,
but not one paired; this, however, might have been due to their habit
of coupling on the wing.[381] Mr. Atkinson could never succeed in India
in making the Tarroo silk-moth breed in confinement.[382] It appears
that a number of moths, especially the Sphingidæ, when hatched in the
autumn out of their proper season, {158} are completely barren; but
this latter case is still involved in some obscurity.[383]

Independently of the fact of many animals under confinement not coupling, or, if they couple, not producing young, there is evidence of another kind, that their sexual functions are thus disturbed. For many cases have been recorded of the loss by male birds when confined of their characteristic plumage. Thus the common linnet (_Linota cannabina_) when caged does not acquire the fine crimson colour on its breast, and one of the buntings (_Emberiza passerina_) loses the black on its head. A Pyrrhula and an Oriolus have been observed to assume the quiet plumage of the hen-bird; and the _Falco albidus_ returned to the dress of an earlier age.[384] Mr. Thomson, the superintendent of the Knowsley menagerie, informed me that he had often observed analogous facts. The horns of a male deer (_Cervus Canadensis_) during the voyage from America were badly developed; but subsequently in Paris perfect horns were produced.

When conception takes place under confinement, the young are often born dead, or die soon, or are ill-formed. This frequently occurs in the Zoological Gardens, and, according to Rengger, with native animals confined in Paraguay. The mother's milk often fails. We may also attribute to the disturbance of the sexual functions the frequent occurrence of that monstrous instinct which leads the mother to devour her own offspring,--a mysterious case of perversion, as it at first appears.

Sufficient evidence has now been advanced to prove that animals when first confined are eminently liable to suffer in their reproductive systems. We feel at first naturally inclined to attribute the result to loss of health, or at least to loss of vigour; but this view can hardly be admitted when we reflect how healthy, long-lived, and vigorous many animals are under {159} captivity, such as parrots, and hawks when used for hawking, chetahs when used for hunting, and elephants. The reproductive organs themselves are not diseased; and the diseases, from which animals in menageries usually perish, are not those which in any way affect their fertility. No domestic animal is more subject too disease than the sheep, yet it is remarkably prolific. The failure of animals to breed under confinement has been sometimes attributed exclusively to a failure in their sexual instincts: this may occasionally come into play, but there is no obvious reason why this instinct should be especially liable to be affected with perfectly tamed animals, except indeed indirectly through the reproductive system itself being disturbed. Moreover, numerous cases have been given of various animals which couple freely under confinement, but never conceive; or, if they conceive and produce young, these are fewer in number than is natural to the species. In the vegetable kingdom instinct of course can play no part; and we shall presently see that plants when removed from their natural conditions are affected in nearly the same manner as animals. Change of climate cannot be the cause of the loss of fertility, for, whilst many animals imported into Europe from extremely different climates breed freely, many others when confined in their native land are completely sterile. Change of food cannot be the chief cause; for ostriches, ducks, and many other animals, which must have undergone a great change in this respect, breed freely. Carnivorous birds when confined are extremely sterile; whilst most carnivorous mammals, except plantigrades, are moderately fertile. Nor can the amount of food be the cause; for a sufficient supply will certainly be given to valuable animals; and there is no reason to suppose that much more food would be given to them, than to our choice domestic productions which retain their full fertility. Lastly, we may infer from the case of the elephant, chetah, various hawks, and of many animals which are allowed to lead an almost free life in their native land, that want of exercise is not the sole cause.

It would appear that any change in the habits of life, whatever these habits may be, if great enough, tends to affect in an inexplicable manner the powers of reproduction. The result {160} depends more on the constitution of the species than on the nature of the change; for certain whole groups are affected more than others; but exceptions always occur, for some species in the most fertile groups refuse to breed, and some in the most sterile groups breed freely. Those animals which usually breed freely under confinement, rarely breed, as I was assured, in the Zoological Gardens, within a year or two after their first importation. When an animal which is generally sterile under confinement happens to breed, the young apparently do not inherit this power; for had this been the case, various quadrupeds and birds, which are valuable for exhibition, would have become common. Dr. Broca even affirms[385] that many animals in the Jardin des Plantes, after having produced young for three or four successive generations, become sterile; but this may be the result of too close interbreeding. It is a remarkable circumstance that many mammals and birds have produced hybrids under confinement quite as readily as, or even more readily than, they have procreated their own kind. Of this fact many instances have been given;[386] and we are thus reminded of those plants which when cultivated refuse to be fertilised by their own pollen, but can easily be fertilised by that of a distinct species. Finally, we must conclude, limited as the conclusion is, that changed conditions of life have an especial power of acting injuriously on the reproductive system. The whole case is quite peculiar, for these organs, though not diseased, are thus rendered incapable of performing their proper functions, or perform them imperfectly.

_Sterility of Domesticated Animals from changed conditions._--With
respect to domesticated animals, as their domestication mainly depends
on the accident of their breeding freely under captivity, we ought not
to expect that their reproductive system would be affected by any
moderate degree of change. Those orders of quadrupeds and birds, of
which the wild species breed most readily in our menageries, have
afforded us the greatest number of domesticated productions. Savages in
most parts of the world are fond of taming animals;[387] and if any of
these regularly produced {161} young, and were at the same time useful,
they would be at once domesticated. If, when their masters migrated
into other countries, they were in addition found capable of
withstanding various climates, they would be still more valuable; and
it appears that the animals which breed readily in captivity can
generally withstand different climates. Some few domesticated animals,
such as the reindeer and camel, offer an exception to this rule. Many
of our domesticated animals can bear with undiminished fertility the
most unnatural conditions; for instance, rabbits, guinea-pigs, and
ferrets breed in miserably confined hutches. Few European dogs of any
kind withstand without degeneration the climate of India; but as long
as they survive, they retain, as I hear from Mr. Falconer, their
fertility; so it is, according to Dr. Daniell, with English dogs taken
to Sierra Leone. The fowl, a native of the hot jungles of India,
becomes more fertile than its parent-stock in every quarter of the
world, until we advance as far north as Greenland and Northern Siberia,
where this bird will not breed. Both fowls and pigeons, which I
received during the autumn direct from Sierra Leone, were at once ready
to couple.[388] I have, also, seen pigeons breeding as freely as the
common kinds within a year after their importation from the Upper Nile.
The guinea-fowl, an aboriginal of the hot and dry deserts of Africa,
whilst living under our damp and cool climate, produces a large supply
of eggs.

Nevertheless, our domesticated animals under new conditions
occasionally show signs of lessened fertility. Roulin asserts that in
the hot valleys of the equatorial Cordillera sheep are not fully
fecund;[389] and according to Lord Somerville,[390] the merino-sheep
which he imported from Spain were not at first perfectly fertile. It is
said[391] that mares brought up on dry food in the stable, and turned
out to grass, do not at first breed. The peahen, as we have seen, is
said not to lay so many eggs in England as in India. It was long before
the canary-bird was fully fertile, and even now first-rate breeding
birds are not common.[392] In the hot and dry province of Delhi, the
eggs of the turkey, as I hear from Dr. Falconer, though placed under a
hen, are extremely liable to fail. According to Roulin, geese taken
within a recent period to the lofty plateau of Bogota, at first laid
seldom, and then only a few eggs; of these scarcely a fourth were
hatched, and half the young birds died: in the second generation they
were more fertile; and when Roulin wrote they were becoming as {162}
fertile as our geese in Europe. In the Philippine Archipelago the
goose, it is asserted, will not breed or even lay eggs.[393] A more
curious case is that of the fowl, which, according to Roulin, when
first introduced would not breed at Cusco in Bolivia, but subsequently
became quite fertile; and the English Game fowl, lately introduced, had
not as yet arrived a its full fertility, for to raise two or three
chickens from a nest of eggs was thought fortunate. In Europe close
confinement has a marked effect on the fertility of the fowl: it has
been found in France that with fowls allowed considerable freedom only
twenty per cent. of the eggs failed; when allowed less freedom forty
per cent. failed; and in close confinement sixty out of the hundred
were not hatched.[394] So we see that unnatural and changed conditions
of life produce some effect on the fertility of our most thoroughly
domesticated animals, in the same manner, though in a far less degree,
as with captive wild animals.

It is by no means rare to find certain males and females which will not
breed together, though both are known to be perfectly fertile with
other males and females. We have no reason to suppose that this is
caused by these animals having been subjected to any change in their
habits of life; therefore such cases are hardly related to our present
subject. The cause apparently lies in an innate sexual incompatibility
of the pair which are matched. Several instances have been communicated
to me by Mr. W. C. Spooner (well known for his essay on
Cross-breeding), by Mr. Eyton of Eyton, by Mr. Wicksted and othe
breeders, and especially by Mr. Waring of Chelsfield, in relation to
horses, cattle, pigs, foxhounds, other dogs, and pigeons.[395] In these
cases, females, which either previously or subsequently were proved to
be fertile, failed to breed with certain males, with whom it was
particularly desired to match them. A change in the constitution of the
female may sometimes have occurred before she was put to the second
male; but in other cases this explanation is hardly tenable, for a
female, known not to be barren, has been unsuccessfully paired seven or
eight times with the same male likewise known to be perfectly fertile.
With cart-mares, which sometimes will not breed with stallions of pure
blood, but subsequently have bred with cart-stallions, Mr. Spooner is
inclined to attribute the failure to the lesser sexual power of the
race-horse. But I have heard from the greatest breeder of race-horses
at the present day, through Mr. Waring, that "it frequently occurs with
a mare to be put several times during one or two seasons to a
particular stallion of acknowledged power, and yet prove barren; the
mare afterwards breeding at once with some other horse." These facts
are worth recording, as they show, like so many previous facts, on what
slight constitutional differences the fertility of an animal often
depends.

{163}

_Sterility of Plants from changed Conditions of Life, and from other causes._

In the vegetable kingdom cases of sterility frequently occur, analogous with those previously given in the animal kingdom. But the subject is obscured by several circumstances, presently to be discussed, namely, the contabescence of the anthers, as Gärtner has named a certain affection--monstrosities--doubleness of the flower--much-enlarged fruit--and long-continued or excessive propagation by buds.

It is notorious that many plants in our gardens and hot-houses, though
preserved in the most perfect health, rarely or never produce seed. I
do not allude to plants which run to leaves, from being kept too damp,
or too warm, or too much manured; for these do not produce the
reproductive individual or flower, and the case may be wholly
different. Nor do I allude to fruit not ripening from want of heat, or
rotting from too much moisture. But many exotic plants, with their
ovules and pollen appearing perfectly sound, will not set any seed. The
sterility in many cases, as I know from my own observation, is simply
due to the absence of the proper insects for carrying the pollen to the
stigma. But after excluding the several cases just specified, there are
many plants in which the reproductive system has been seriously
affected by the altered conditions of life to which they have been
subjected.

It would be tedious to enter on many details. Linnæus long ago
observed[396] that Alpine plants, although naturally laded with seed,
produce either few or none when cultivated in gardens. But exceptions
often occur: the _Draba sylvestris_, one of our most thoroughly Alpine
plants, multiplies itself by seed in Mr. H. C. Watson's garden, near
London; and Kerner, who has particularly attended to the cultivation of
Alpine plants, found that various kinds, when cultivated, spontaneously
sowed themselves.[397] Many plants which naturally grow in peat-earth
are entirely sterile in our gardens. I have noticed the same fact with
several liliaceous plants, which nevertheless grew vigorously.

Too much manure renders some kinds utterly sterile, as I have myself
observed. The tendency to sterility from this cause runs in families;
thus, according to Gärtner,[398] it is hardly possible to give too much
manure to most Gramineæ, Cruciferæ, and Leguminosæ, whilst succulent
and bulbous-rooted plants are easily affected. Extreme poverty of soil
is less {164} apt to induce sterility; but dwarfed plants of _Trifolium
minus_ and _repens_, growing on a lawn often mown and never manured,
did not produce any seed. The temperature of the soil, and the season
at which plants are watered, often have a marked effect on their
fertility, as was observed by Kölreuter in the case of Mirabilis.[399]
Mr. Scott in the Botanic Gardens of Edinburgh observed that _Oncidium
divaricatum_ would not set seed when grown in a basket in which it
throve, but was capable of fertilisation in a pot where it was a little
damper. _Pelargonium fulgidum_, for many years after its introduction,
seeded freely; it then became sterile; now it is fertile[400] if kept
in a dry stove during the winter. Other varieties of pelargonium are
sterile and others fertile without our being able to assign any cause.
Very slight changes in the position of a plant, whether planted on a
bank or at its base, sometimes make all the difference in its producing
seed. Temperature apparently has a much more powerful influence on the
fertility of plants than on that of animals. Nevertheless it is
wonderful what changes some few plants will withstand with undiminished
fertility: thus the _Zephyranthes candida_, a native of the moderately
warm banks of the Plata, sows itself in the hot dry country near Lima,
and in Yorkshire resists the severest frosts, and I have seen seeds
gathered from pods which had been covered with snow during three
weeks.[401] _Berberis Wallichii_, from the hot Khasia range in India,
is uninjured by our sharpest frosts, and ripens its fruit under our
cool summers. Nevertheless I presume we must attribute to change of
climate the sterility of many foreign plants; thus the Persian and
Chinese lilacs (_Syringa Persica_ and _Chinensis_), though perfectly
hardly, never here produce a seed; the common lilac (_S. vulgaris_)
seeds with us moderately well, but in parts of Germany the capsules
never contain seed.[402]

Some of the cases, given in the last chapter, of self-impotent plants,
which are fertile both on the male and female side when united with
distinct individuals or species, might have been here introduced; for
as this peculiar form of sterility generally occurs with exotic plants
or with endemic plants cultivated in pots, and as it disappeared in the
_Passiflora alata_ when grafted, we may conclude that in these cases it
is the result of the treatment to which the plants or their parents
have been exposed.

The liability of plants to be affected in their fertility by slightly
changed conditions is the more remarkable, as the pollen when once in
process of formation is not easily injured; a plant may be
transplanted, or a branch with flower-buds be cut off and placed in
water, and the pollen will be matured. Pollen, also, when once mature,
may be kept for weeks or even months.[403] The female organs are more
sensitive, for Gärtner[404] found that dicotyledonous plants, when
carefully removed so that they did not in the least flag, could seldom
be fertilised; this occurred even with potted {165} plants if the roots
had grown out of the hole at the bottom. In some few cases, however, as
with Digitalis, transplantation did not prevent fertilisation; and
according to the testimony of Mawz, _Brassica rapa_, when pulled up by
its roots and placed in water, ripened its seed. Flower-stems of
several monocotyledonous plants when cut off and placed in water
likewise produce seed. But in these cases I presume that the flowers
had been already fertilised, for Herbert[405] found with the Crocus
that the plants might be removed or mutilated after the act of
fertilisation, and would still perfect their seeds; but that, if
transplanted before being fertilised, the application of pollen was
powerless.

Plants which have been long cultivated can generally endure with
undiminished fertility various and great changes; but not in most cases
so great a change of climate as domesticated animals. It is remarkable
that many plants under these circumstances are so much affected that
the proportions and the nature of their chemical ingredients are
modified, yet their fertility is unimpaired. Thus, as Dr. Falconer
informs me, there is a great difference in the character of the fibre
in hemp, in the quantity of oil in the seed of the Linum, in the
proportion of narcotin to morphine in the poppy, in gluten to starch in
wheat, when these plants are cultivated on the plains and on the
mountains of India; nevertheless, they all remain fully fertile.

_Contabescence._--Gärtner has designated by this term a peculiar
condition of the anthers in certain plants, in which they are
shrivelled, or become brown and tough, and contain no good pollen. When
in this state they exactly resemble the anthers of the most sterile
hybrids. Gärtner,[406] in his discussion on this subject, has shown
that plants of many orders are occasionally thus affected; but the
Caryophyllaceæ and Liliaceæ suffer most, and to these orders, I think,
the Ericaceæ may be added. Contabescence varies in degree, but on the
same plant all the flowers are generally affected to nearly the same
extent. The anthers are affected at a very early period in the
flower-bud, and remain in the same state (with one recorded exception)
during the life of the plant. The affection cannot be cured by any
change of treatment, and is propagated by layers, cuttings, &c., and
perhaps even by seed. In contabescent plants the female organs are
seldom affected, or merely become precocious in their development. The
cause of this affection is doubtful, and is different in different
cases. Until I read Gärtner's discussion I attributed it, as apparently
did Herbert, to the unnatural treatment of the plants; but its
permanence under changed conditions, and the female organs not being
affected, seem incompatible with this view. The fact of several endemic
plants becoming contabescent in our gardens seems, at first sight,
equally incompatible with this view; but Kölreuter believes that this
is the result of their transplantation. The contabescent plants of
Dianthus and Verbascum, found wild by Wiegmann, grew on a dry and
sterile bank. The fact that exotic {166} plants are eminently liable to
this affection also seems to show that it is in some manner caused by
their unnatural treatment. In some instances, as with Silene, Gärtner's
view seems the most probable, namely, that it is caused by an inherent
tendency in the species to become dioecious. I can add another cause,
namely, the illegitimate unions of reciprocally dimorphic or trimorphic
plants, for I have observed seedlings of three species of Primula and
of _Lythrum salicaria_, which had been raised from plants
illegitimately fertilised by their own-form pollen, with some or all
their anthers in a contabescent state. There is perhaps an additional
cause, namely, self-fertilisation; for many plants of Dianthus and
Lobelia, which had been raised from self-fertilised seeds, had their
anthers in this state; but these instances are not conclusive, as both
genera are liable from other causes to this affection.

Cases of an opposite nature likewise occur, namely, plants with the
female organs struck with sterility, whilst the male organs remain
perfect. _Dianthus Japonicus_, a Passiflora, and Nicotiana, have been
described by Gärtner[407] as being in this unusual condition.

_Monstrosities as a cause of Sterility._--Great deviations of
structure, even when the reproductive organs themselves are not
seriously affected, sometimes cause plants to become sterile. But in
other cases plants may become monstrous to an extreme degree and yet
retain their full fertility. Gallesio, who certainly had great
experience,[408] often attributes sterility to this cause; but it may
be suspected that in some of his cases sterility was the cause, and not
the result, of the monstrous growths. The curious St. Valery apple,
although it bears fruit, rarely produces seed. The wonderfully
anomalous flowers of _Begonia frigida_, formerly described, though they
appear fit for fructification, are sterile.[409] Species of Primulæ, in
which the calyx is brightly coloured, are said[410] to be often
sterile, though I have known them to be fertile. On the other hand,
Verlot gives several cases of proliferous flowers which can be
propagated by seed. This was the case with a poppy, which had become
monopetalous by the union of its petals.[411] Another extraordinary
poppy, with the stamens replaced by numerous small supplementary
capsules, likewise reproduces itself by seed. This has also occurred
with a plant of _Saxifraga geum_, in which a series of adventitious
carpels, bearing ovules on their margins, had been developed between
the stamens and the normal carpels.[412] Lastly, with respect to
peloric flowers, which depart wonderfully from the natural
structure,--those of _Linaria vulgaris_ seem generally to be more or
less sterile, whilst those before described of _Antirrhinum majus_,
when artificially fertilised with their own pollen, are perfectly {167}
fertile, though sterile when left to themselves, for bees are unable to
crawl into the narrow tubular flower. The peloric flowers of _Corydalis
solida_, according to Godron,[413] are barren; whilst those of Gloxinia
are well known to yield plenty of seed. In our greenhouse Pelargoniums,
the central flower of the truss is often peloric, and Mr. Masters
informs me that he tried in vain during several years to get seed from
these flowers. I likewise made many vain attempts, but sometimes
succeeded in fertilising them with pollen from a normal flower of
another variety; and conversely I several times fertilised ordinary
flowers with peloric pollen. Only once I succeeded in raising a plant
from a peloric flower fertilised by pollen from a peloric flower borne
by another variety; but the plant, it may be added, presented nothing
particular in its structure. Hence we may conclude that no general rule
can be laid down; but any great deviation from the normal structure,
even when the reproductive organs themselves are not seriously
affected, certainly often leads to sexual impotence.

_Double Flowers._--When the stamens are converted into petals, the
plant becomes on the male side sterile; when both stamens and pistils
are thus changed, the plant becomes completely barren. Symmetrical
flowers having numerous stamens and petals are the most liable to
become double, as perhaps follows from all multiple organs being the
most subject to variability. But flowers furnished with only a few
stamens, and others which are asymmetrical in structure, sometimes
become double, as we see with the double gorse or Ulex, Petunia, and
Antirrhinum. The Compositæ bear what are called double flowers by the
abnormal development of the corolla of their central florets.
Doubleness is sometimes connected with prolification,[414] or the
continued growth of the axis of the flower. Doubleness is strongly
inherited. No one has produced, as Lindley remarks,[415] double flowers
by promoting the perfect health of the plant. On the contrary,
unnatural conditions of life favour their production. There is some
reason to believe that seeds kept during many years, and seeds believed
to be imperfectly fertilised, yield double flowers more freely than
fresh and perfectly fertilised seed.[416] Long-continued cultivation in
rich soil seems to be the commonest exciting cause. A double narcissus
and a double _Anthemis nobilis_, transplanted into very poor soil, have
been observed to become single;[417] and I have seen a completely
double white primrose rendered permanently single by being divided and
transplanted whilst in full flower. It has been observed by Professor
Morren that doubleness of the flowers and variegation of the leaves are
antagonistic states; but so many exceptions to the rule have lately
been recorded,[418] that, though general, it cannot be looked at as
invariable. {168} Variegation seems generally to result from a feeble
or atrophied condition of the plant, and a large proportion of the
seedlings raised from parents both of which are variegated usually
perish at an early age; hence we may perhaps infer that doubleness,
which is the antagonistic state, commonly arises from a plethoric
condition. On the other hand, extremely poor soil sometimes, though
rarely, appears to cause doubleness: I formerly described[419] some
completely double, bud-like, flowers produced in large numbers by
stunted wild plants of _Gentiana amarella_ growing on a poor chalky
bank. I have also noticed a distinct tendency to doubleness in the
flowers of a Ranunculus, Horse-chesnut, and Bladder-nut (_Ranunculus
repens_, _Æsculus pavia_, and _Staphylea_), growing under very
unfavourable conditions. Professor Lehman[420] found several wild
plants growing near a hot spring with double flowers. With respect to
the cause of doubleness, which arises, as we see, under widely
different circumstances, I shall presently attempt to show that the
most probable view is that unnatural conditions first give a tendency
to sterility, and that then, on the principle of compensation, as the
reproductive organs do not perform their proper functions, they either
become developed into petals, or additional petals are formed. This
view has lately been supported by Mr. Laxton,[421] who advances the
case of some common peas, which, after long-continued heavy rain,
flowered a second time, and produced double flowers.

_Seedless Fruit._--Many of our most valuable fruits, although
consisting in a homological sense of widely different organs, are
either quite sterile, or produce extremely few seeds. This is
notoriously the case with our best pears, grapes, and figs, with the
pine-apple, banana, bread-fruit, pomegranate, azarole, date-palms, and
some members of the orange-tribe. Poorer varieties of these same fruits
either habitually or occasionally yield seed.[422] Most horticulturists
look at the great size and anomalous development of the fruit as the
cause, and sterility as the result; but the opposite view, as we shall
presently see, is more probable.

_Sterility from the excessive development of the Organs of Growth or
Vegetation._--Plants which from any cause grow too luxuriantly, and
produce leaves, stems, runners, suckers, tubers, bulbs, &c., in excess,
sometimes do not flower, or if they flower do not yield seed. To make
European vegetables under the hot climate of India yield seed, it is
necessary to check their growth; and, when one-third grown, they are
taken up, and their stems and {169} tap-roots are cut or
mutilated.[423] So it is with hybrids; for instance, Prof. Lecoq[424]
had three plants of Mirabilis, which, though they grew luxuriantly and
flowered, were quite sterile; but after beating one with a stick until
a few branches alone were left, these at once yielded good seed. The
sugar-cane, which grows vigorously and produces a large supply of
succulent stems, never, according to various observers, bears seed in
the West Indies, Malaga, India, Cochin China, or the Malay
Archipelago.[425] Plants which produce a large number of tubers are apt
to be sterile, as occurs, to a certain extent, with the common potato;
and Mr. Fortune informs me that the sweet potato (_Convolvulus
batatas_) in China never, as far as he has seen, yields seed. Dr. Royle
remarks[426] that in India the _Agave vivipara_, when grown in rich
soil, invariably produces bulbs, but no seeds; whilst a poor soil and
dry climate leads to an opposite result. In China, according to Mr.
Fortune, an extraordinary number of little bulbs are developed in the
axils of the leaves of the yam, and this plant does not bear seed.
Whether in these cases, as in those of double flowers and seedless
fruit, sexual sterility from changed conditions of life is the primary
cause which leads to the excessive development of the organs of
vegetation, is doubtful; though some evidence might be advanced in
favour of this view. It is perhaps a more probable view that plants
which propagate themselves largely by one method, namely by buds, have
not sufficient vital power or organised matter for the other method of
sexual generation.

Several distinguished botanists and good practical judges believe that
long-continued propagation by cuttings, runners, tubers, bulbs, &c.,
independently of any excessive development of these parts, is the cause
of many plants failing to produce flowers and of others failing to
produce fertile flowers,--it is as if they had lost the habit of sexual
generation.[427] That many plants when thus propagated are sterile
there can be no doubt, but whether the long continuance of this form of
propagation is the actual cause of their sterility, I will not venture,
from the want of sufficient evidence, to express an opinion.

That plants may be propagated for long periods by buds, without the aid
of sexual generation, we may safely infer from this being the case with
many plants which must have long survived in a state of nature. As I
have had occasion before to allude to this subject, I will here give
such cases as I have collected. Many alpine plants ascend mountains
beyond the height at which they can produce seed.[428] Certain species
of {170} Poa and Festuca, when growing on mountain-pastures, propagate
themselves, as I hear from Mr. Bentham, almost exclusively by bulblets.
Kalm gives a more curious instance[429] of several American trees,
which grow so plentifully in marshes or in thick woods, that they are
certainly well adapted for these stations, yet scarcely ever produce
seeds; but when accidentally growing on the outside of the marsh or
wood, are loaded with seed. The common ivy is found in Northern Sweden
and Russia, but flowers and fruits only in the southern provinces. The
_Acorus calamus_ extends over a large portion of the globe, but so
rarely perfects its fruit that this has been seen but by few
botanists.[430] The _Hypericum calycinum_, which propagates itself so
freely in our shrubberies by rhizomas and is naturalised in Ireland,
blossoms profusely, but sets no seed; nor did it set any when
fertilised in my garden by pollen from plants growing at a distance.
The _Lysimachia nummularia_, which is furnished with long runners, so
seldom produces seed-capsules, that Prof. Decaisne,[431] who has
especially attended to this plant, has never seen it in fruit. The
_Carex rigida_ often fails to perfect its seed in Scotland, Lapland,
Greenland, Germany, and New Hampshire in the United States.[432] The
periwinkle (_Vinca minor_), which spreads largely by runners, is said
scarcely ever to produce fruit in England;[433] but this plant requires
insect-aid for its fertilisation, and the proper insects may be absent
or rare. The _Jussiæa grandiflora_ has become naturalised in Southern
France, and has spread by its rhizomas so extensively as to impede the
navigation of the waters, but never produces fertile seed.[434] The
horse-radish (_Cochlearia armoracia_) spreads pertinaciously and is
naturalised in various parts of Europe; though it bears flowers, these
rarely produce capsules: Professor Caspary also informs me that he has
watched this plant since 1851, but has never seen its fruit; nor is
this surprising, as he finds scarcely a grain of good pollen. The
common little _Ranunculus ficaria_ rarely, and some say never, bears
seed in England, France, or Switzerland; but in 1863 I observed seeds
on several plants growing near my house. According to M. Chatin, there
are two forms of this Ranunculus; and it is the bulbiferous form which
does not yield seed from producing no pollen.[435] Other cases {171}
analogous with the foregoing could be given; for instance, some kinds
of mosses and lichens have never been seen to fructify in France.

Some of these endemic and naturalised plants are probably rendered
sterile from excessive multiplication by buds, and their consequent
incapacity to produce and nourish seed. But the sterility of others
more probably depends on the peculiar conditions under which they live,
as in the case of the ivy in the northern parts of Europe, and of the
trees in the swamps of the United States; yet these plants must be in
some respects eminently well adapted for the stations which they
occupy, for they hold their places against a host of competitors.

Finally, when we reflect on the sterility which accompanies the doubling of flowers,--the excessive development of fruit,--and a great increase in the organs of vegetation, we must bear in mind that the whole effect has seldom been caused at once. An incipient tendency is observed, and continued selection completes the work, as is known to be the case with our double flowers and best fruits. The view which seems the most probable, and which connects together all the foregoing facts and brings them within our present subject, is, that changed and unnatural conditions of life first give a tendency to sterility; and in consequence of this, the organs of reproduction being no longer able fully to perform their proper functions, a supply of organised matter, not required for the development of the seed, flows either into these same organs and renders them foliaceous, or into the fruit, stems, tubers, &c., increasing their size and succulency. But I am far from wishing to deny that there exists, independently of any incipient sterility, an antagonism between the two forms of reproduction, namely, by seed and by buds, when either is carried to an extreme degree. That incipient sterility plays an important part in the doubling of flowers, and in the other cases just specified, I infer chiefly from the following facts. When fertility is lost from a wholly different cause, namely, from hybridism, there is a strong tendency, as Gärtner[436] affirms, for flowers to become double, and this tendency is inherited. Moreover it is notorious that with hybrids the male organs become sterile before the female organs, and with double flowers the stamens first become {172} foliaceous. This latter fact is well shown by the male flowers of dioecious plants, which, according to Gallesio,[437] first become double. Again, Gärtner[438] often insists that the flowers of even utterly sterile hybrids, which do not produce any seed, generally yield perfect capsules or fruit,--a fact which has likewise been repeatedly observed by Naudin with the Cucurbitaceæ; so that the production of fruit by plants rendered sterile through any other and distinct cause is intelligible. Kölreuter has also expressed his unbounded astonishment at the size and development of the tubers in certain hybrids; and all experimentalists[439] have remarked on the strong tendency in hybrids to increase by roots, runners, and suckers. Seeing that hybrid plants, which from their nature are more or less sterile, thus tend to produce double flowers; that they have the parts including the seed, that is the fruit, perfectly developed, even when containing no seed; that they sometimes yield gigantic roots; that they almost invariably tend to increase largely by suckers and other such means;--seeing this, and knowing, from the many facts given in the earlier parts of this chapter, that almost all organic beings when exposed to unnatural conditions tend to become more or less sterile, it seems much the most probable view that with cultivated plants sterility is the exciting cause, and double flowers, rich seedless fruit, and in some cases largely-developed organs of vegetation, &c., are the indirect results--these results having been in most cases largely increased through continued selection by man.

* * * * *

{173}