Chapter XVI

CAUSES WHICH INTERFERE WITH THE FREE CROSSING OF VARIETIES--INFLUENCE OF DOMESTICATION ON FERTILITY.

DIFFICULTIES IN JUDGING OF THE FERTILITY OF VARIETIES WHEN
CROSSED--VARIOUS CAUSES WHICH KEEP VARIETIES DISTINCT, AS THE PERIOD OF
BREEDING AND SEXUAL PREFERENCE--VARIETIES OF WHEAT SAID TO BE STERILE
WHEN CROSSED--VARIETIES OF MAIZE, VERBASCUM, HOLLYHOCK, GOURDS, MELONS,
AND TOBACCO, RENDERED IN SOME DEGREE MUTUALLY STERILE--DOMESTICATION
ELIMINATES THE TENDENCY TO STERILITY NATURAL TO SPECIES WHEN
CROSSED--ON THE INCREASED FERTILITY OF UNCROSSED ANIMALS AND PLANTS
FROM DOMESTICATION AND CULTIVATION.

The domesticated races of both animals and plants, when crossed, are with extremely few exceptions quite prolific,--in some cases even more so than the purely bred parent-races. The offspring, also, raised from such crosses are likewise, as we shall see in the following chapter, generally more vigorous and fertile than their parents. On the other hand, species when crossed, and their hybrid offspring, are almost invariability in some degree sterile; and here there seems to exist a broad and insuperable distinction between races and species. The importance of this subject as bearing on the origin of species is obvious; and we shall hereafter recur to it.

It is unfortunate how few precise observations have been made on the fertility of mongrel animals and plants during several successive generations. Dr. Broca[214] has remarked that no one has observed whether, for instance, mongrel dogs, bred _inter se_, are indefinitely fertile; yet, if a shade of infertility be detected by careful observation in the offspring of natural forms when crossed, it is thought that their specific distinction is proved. But so many breeds of sheep, cattle, pigs, dogs, and poultry, have been crossed and recrossed in various ways, that any sterility, if it had existed, would from being injurious {101} almost certainly have been observed. In investigating the fertility of crossed varieties many sources of doubt occur. Whenever the least trace of sterility between two plants, however closely allied, was observed by Kölreuter, and more especially by Gärtner, who counted the exact number of seed in each capsule, the two forms were at once ranked as distinct species; and if this rule be followed, assuredly it will never be proved that varieties when crossed are in any degree sterile. We have formerly seen that certain breeds of dogs do not readily pair together; but no observations have been made whether, when paired, they produce the full number of young, and whether the latter are perfectly fertile _inter se_; but, supposing that some degree of sterility were found to exist, naturalists would simply infer that these breeds were descended from aboriginally distinct species; and it would be scarcely possible to ascertain whether or not this explanation was the true one.

The Sebright Bantam is much less prolific than any other breed of fowls, and is descended from a cross between two very distinct breeds, recrossed by a third sub-variety. But it would be extremely rash to infer that the loss of fertility was in any manner connected with its crossed origin, for it may with more probability be attributed either to long-continued close interbreeding, or to an innate tendency to sterility correlated with the absence of hackles and sickle tail-feathers.

Before giving the few recorded cases of forms, which must be ranked as varieties, being in some degree sterile when crossed, I may remark that other causes sometimes interfere with varieties freely intercrossing. Thus they may differ too greatly in size, as with some kinds of dogs and fowls: for instance, the editor of the 'Journal of Horticulture, &c.,'[215] says that he can keep Bantams with the larger breeds without much danger of their crossing, but not with the smaller breeds, such as Games, Hamburgs, &c. With plants a difference in the period of flowering serves to keep varieties distinct, as with the various kinds of maize and wheat: thus Colonel Le Couteur[216] remarks, "the Talavera wheat, from flowering much earlier than any other kind, is sure to continue pure." In different parts of {102} the Falkland Islands the cattle are breaking up into herds of different colours; and those on the higher ground, which are generally white, usually breed, as I am informed by Admiral Sulivan, three months earlier than those on the lowlands; and this would manifestly tend to keep the herds from blending.

Certain domestic races seem to prefer breeding with their own kind; and this is a fact of some importance, for it is a step towards that instinctive feeling which helps to keep closely allied species in a state of nature distinct. We have now abundant evidence that, if it were not for this feeling, many more hybrids would be naturally produced than is the case. We have seen in the first chapter that the alco dog of Mexico dislikes dogs of other breeds; and the hairless dog of Paraguay mixes less readily with the European races, than the latter do with each other. In Germany the female Spitz-dog is said to receive the fox more readily than will other dogs; a female Australian Dingo in England attracted the wild male foxes. But these differences in the sexual instinct and attractive power of the various breeds may be wholly due to their descent from distinct species. In Paraguay the horses have much freedom, and an excellent observer[217] believes that the native horses of the same colour and size prefer associating with each other, and that the horses which have been imported from Entre Rios and Banda Oriental into Paraguay likewise prefer associating together. In Circassia six sub-races of the horse are known and have received distinct names; and a native proprietor of rank[218] asserts that horses of three of these races, whilst living a free life, almost always refuse to mingle and cross, and will even attack each other.

It has been observed, in a district stocked with heavy Lincolnshire and light Norfolk sheep, that both kinds, though bred together, when turned out, "in a short time separate to a sheep;" the Lincolnshires drawing off to the rich soil, and the Norfolks to their own dry light soil; and as long as there is plenty of grass, "the two breeds keep themselves as distinct as rooks and pigeons." In this case different habits of {103} life tend to keep the races distinct. On one of the Faroe islands, not more than half a mile in diameter, the half-wild native black sheep are said not to have readily mixed with the imported white sheep. It is a more curious fact that the semi-monstrous ancon sheep of modern origin "have been observed to keep together, separating themselves from the rest of the flock, when put into enclosures with other sheep."[219] With respect to fallow deer, which live in a semi-domesticated condition, Mr. Bennett[220] states that the dark and pale coloured herds, which have long been kept together in the Forest of Dean, in High Meadow Woods, and in the New Forest, have never been known to mingle: the dark-coloured deer, it may be added, are believed to have been first brought by James I. from Norway, on account of their greater hardiness. I imported from the island of Porto Santo two of the feral rabbits, which differ, as described in the fourth chapter, from common rabbits; both proved to be males, and, though they lived during some years in the Zoological Gardens, the superintendent, Mr. Bartlett, in vain endeavoured to make them breed with various tame kinds; but whether this refusal to breed was due to any change in instinct, or simply to their extreme wildness; or whether confinement had rendered them sterile, as often occurs, cannot be told.

Whilst matching for the sake of experiment many of the most distinct breeds of pigeons, it frequently appeared to me that the birds, though faithful to their marriage vow, retained some desire after their own kind. Accordingly I asked Mr. Wicking, who has kept a larger stock of various breeds together than any man in England, whether he thought that they would prefer pairing with their own kind, supposing that there were males and females enough of each; and he without hesitation answered that he was convinced that this was the case. It has often been noticed that the dovecot pigeon seems to have an actual aversion towards the several fancy breeds;[221] yet all have {104} certainly sprung from a common progenitor. The Rev. W. D. Fox informs me that his flocks of white and common Chinese geese kept distinct.

These facts and statements, though some of them are incapable of proof, resting only on the opinion of experienced observers, show that some domestic races are led by different habits of life to keep to a certain extent separate, and that others prefer coupling with their own kind, in the same manner as species in a state of nature, though in a much less degree.

With respect to sterility from the crossing of domestic races, I know
of no well-ascertained case with animals. This fact, seeing the great
difference in structure between some breeds of pigeons, fowls, pigs,
dogs, &c., is extraordinary, in contrast with the sterility of many
closely allied natural species when crossed; but we shall hereafter
attempt to show that it is not so extraordinary as it at first appears.
And it may be well here to recall to mind that the amount of external
difference between two species will not safely guide us in foretelling
whether or not they will breed together,--some closely allied species
when crossed being utterly sterile, and others which are extremely
unlike being moderately fertile. I have said that no case of sterility
in crossed races rests on satisfactory evidence; but here is one which
at first seems trustworthy. Mr. Youatt,[222] and a better authority
cannot be quoted, states, that formerly in Lancashire crosses were
frequently made between longhorn and shorthorn cattle; the first cross
was excellent, but the produce was uncertain; in the third or fourth
generation the cows were bad milkers; "in addition to which, there was
much uncertainty whether the cows would conceive; and full one-third of
the cows among some of these half-breds failed to be in calf." This at
first seems a good case; but Mr. Wilkinson states,[223] that a breed
derived from this same cross was actually established in another part
of England; and if it had failed in fertility, the fact would surely
have been noticed. Moreover, supposing that Mr. Youatt had proved his
case, it might be argued that the sterility was wholly due to the two
parent-breeds being descended from primordially distinct species.

I will give a case with plants, to show how difficult it is to get
sufficient evidence. Mr. Sheriff, who has been so successful in the
formation of new races of wheat, fertilised the Hopetoun with the
Talavera; in the first and second generations the produce was
intermediate in character, but in the fourth generation "it was found
to consist of many varieties; nine-tenths of the florets proved barren,
and many of the seeds seemed shrivelled abortions, void of vitality,
and the whole race was evidently verging to extinction."[224] Now,
considering how little these {105} varieties of wheat differ in any
important character, it seems to me very improbable that the sterility
resulted, as Mr. Sheriff thought, from the cross, but from some quite
distinct cause. Until such experiments are many times repeated, it
would be rash to trust them; but unfortunately they have been rarely
tried even once with sufficient care.

Gärtner has recorded a more remarkable and trustworthy case: he
fertilised thirteen panicles (and subsequently nine others) on a dwarf
maize bearing yellow seed[225] with pollen of a tall maize having red
seed; and one head alone produced good seed, only five in number.
Though these plants are monoecious, and therefore do not require
castration, yet I should have suspected some accident in the
manipulation had not Gärtner expressly stated that he had during many
years grown these two varieties together, and they did not
spontaneously cross; and this, considering that the plants are
monoecious and abound with pollen, and are well known generally to
cross freely, seems explicable only on the belief that these two
varieties are in some degree mutually infertile. The hybrid plants
raised from the above five seed were intermediate in structure,
extremely variable, and perfectly fertile.[226] No one, I believe, has
hitherto suspected that these varieties of maize are distinct species;
but had the hybrids been in the least sterile, no doubt Gärtner would
at once have so classed them. I may here remark, that with undoubted
species there is not necessarily any close relation between the
sterility of a first cross and that of the hybrid offspring. Some
species can be crossed with facility, but produce utterly sterile
hybrids; others can be crossed with extreme difficulty, but the hybrids
when produced are moderately fertile. I am not aware, however, of any
instance quite like this of the maize with natural species, namely, of
a first cross made with difficulty, but yielding perfectly fertile
hybrids.

The following case is much more remarkable, and evidently perplexed
Gärtner, whose strong wish it was to draw a broad line of distinction
between species and varieties. In the genus Verbascum, he made, during
eighteen years, a vast number of experiments, and crossed no less than
1085 flowers and counted their seeds. Many of these experiments
consisted in crossing white and yellow varieties of both _V. lychnitis_
and _V. blattaria_ with nine other species and their hybrids. That the
white and yellow flowered plants of these two species are really
varieties, no one has doubted; and Gärtner actually raised in the case
of both species one variety from the seed of the other. Now in two of
his works[227] he distinctly asserts that crosses between
similarly-coloured flowers yield more seed than between
dissimilarly-coloured; so that the yellow-flowered variety of either
species (and conversely with the white-flowered variety), when crossed
with pollen of its own kind, yields more seed than when crossed with
that of the white variety; and so it is when differently coloured
species are crossed. The general results may be seen in the Table at
the {106} end of his volume. In one instance he gives[228] the
following details; but I must premise that Gärtner, to avoid
exaggerating the degree of sterility in his crosses, always compares
the _maximum_ number obtained from a cross with the _average_ number
naturally given by the pure mother-plant. The white-variety of _V.
lychnitis_, naturally fertilised by its own pollen, gave from an
_average_ of twelve capsules ninety-six good seeds in each; whilst
twenty flowers fertilised with pollen from the yellow variety of this
same species, gave as the _maximum_ only eighty-nine good seed; so that
we have the proportion of 1000 to 908, according to Gärtner's usual
scale. I should have thought it possible that so small a difference in
fertility might have been accounted for by the evil effects of the
necessary castration; but Gärtner shows that the white variety of _V.
lychnitis_, when fertilised first by the white variety of _V.
blattaria_, and then by the yellow variety of this species, yielded
seed in the proportion of 622 to 438; and in both these cases
castration was performed. Now the sterility which results from the
crossing of the differently coloured varieties of the same species, is
fully as great as that which occurs in many cases when distinct species
are crossed. Unfortunately Gärtner compared the results of the first
unions alone, and not the sterility of the two sets of hybrids produced
from the white variety of _V. lychnitis_ when fertilised by the white
and yellow varieties of _V. blattaria_, for it is probable that they
would have differed in this respect.

Mr. J. Scott has given me the results of a series of experiments on
Verbascum, made by him in the Botanic Gardens of Edinburgh. He repeated
some of Gärtner's experiments on distinct species, but obtained only
fluctuating results; some confirmatory, but the greater number
contradictory; nevertheless these seem hardly sufficient to overthrow
the conclusions arrived at by Gärtner from experiments tried on a much
larger scale. In the second place Mr. Scott experimented on the
relative fertility of unions between similarly and
dissimilarly-coloured varieties of the same species. Thus he fertilised
six flowers of the yellow variety of _V. lychnitis_ by its own pollen,
and obtained six capsules, and calling, for the sake of having a
standard of comparison, the average number of good seed in each one
hundred, he found that this same yellow variety, when fertilised by the
white variety, yielded from seven capsules an average of ninety-four
seed. On the same principle, the white variety of _V. lychnitis_ by its
own pollen (from six capsules), and by the pollen of the yellow variety
(eight capsules), yielded seed in the proportion of 100 to 82. The
yellow variety of _V. thapsus_ by its own pollen (eight capsules), and
by that of the white variety (only two capsules), yielded seed in the
proportion of 100 to 94. Lastly, the white variety of _V. blattaria_ by
its own pollen (eight capsules), and by that of the yellow variety
(five capsules), yielded seed in the proportion of 100 to 79. So that
in every case the unions of dissimilarly-coloured varieties of the same
species were less fertile than the unions of similarly-coloured
varieties; when all the cases are grouped together, the difference of
fertility is as 86 to 100. Some additional trials were made, and
altogether thirty-six similarly-coloured unions yielded thirty-five
good {107} capsules; whilst thirty-five dissimilarly-coloured unions
yielded only twenty-six good capsules. Besides the foregoing
experiments, the purple _V. phoeniceum_ was crossed by a rose-coloured
and a white variety of the same species; these two varieties were also
crossed together, and these several unions yielded less seed than _V.
phoeniceum_ by its own pollen. Hence it follows from Mr. Scott's
experiments, that in the genus Verbascum the similarly and
dissimilarly-coloured varieties of the same species behave, when
crossed, like closely allied but distinct species.[229]

This remarkable fact of the sexual affinity of similarly-coloured
varieties, as observed by Gärtner and Mr. Scott, may not be of very
rare occurrence; for the subject has not been attended to by others.
The following case is worth giving, partly to show how difficult it is
to avoid error. Dr. Herbert[230] has remarked that variously-coloured
double varieties of the hollyhock (_Althæa rosea_) may be raised with
certainty by seed from plants growing close together. I have been
informed that nurserymen who raise seed for sale do not separate their
plants; accordingly I procured seed of eighteen named varieties; of
these, eleven varieties produced sixty-two plants all perfectly true to
their kind; and seven produced forty-nine plants, half of which were
true and half false. Mr. Masters of Canterbury has given me a more
striking case; he saved seed from a great bed of twenty-four named
varieties planted in closely adjoining rows, and each variety
reproduced itself truly with only sometimes a shade of difference in
tint. Now in the hollyhock the pollen, which is abundant, is matured
and nearly all shed before the stigma of the same flower is ready to
receive it;[231] and as bees covered with pollen incessantly fly from
plant to plant, it would appear that adjoining varieties could not
escape being crossed. As, however, this does not occur, it appeared to
me probable that the pollen {108} of each variety was prepotent on its
own stigma over that of all other varieties. But Mr. C. Turner of
Slough, well known for his success in the cultivation of this plant,
informs me that it is the doubleness of the flowers which prevents the
bees gaining access to the pollen and stigma; and he finds that it is
difficult even to cross them artificially. Whether this explanation
will fully account for varieties in close proximity propagating
themselves so truly by seed, I do not know.

The following cases are worth giving, as they relate to monoecious
forms, which do not require, and consequently have not been injured by,
castration. Girou de Buzareingues crossed what he designates three
varieties of gourd,[232] and asserts that their mutual fertilisation is
less easy in proportion to the difference which they present. I am
aware how imperfectly the forms in this group were until recently
known; but Sageret,[233] who ranked them according to their mutual
fertility, considers the three forms above alluded to as varieties, as
does a far higher authority, namely, M. Naudin.[234] Sageret[235] has
observed that certain melons have a greater tendency, whatever the
cause may be, to keep true than others; and M. Naudin, who has had such
immense experience in this group, informs me that he believes that
certain varieties intercross more readily than others of the same
species; but he has not proved the truth of this conclusion; the
frequent abortion of the pollen near Paris being one great difficulty.
Nevertheless, he has grown close together, during seven years, certain
forms of Citrullus, which, as they could be artificially crossed with
perfect facility and produced fertile offspring, are ranked as
varieties; but these forms when not artificially crossed kept true.
Many other varieties, on the other hand, in the same group cross with
such facility, as M. Naudin repeatedly insists, that without being
grown far apart they cannot be kept in the least true.

Another case, though somewhat different, may be here given, as it is
highly remarkable, and is established on excellent evidence. Kölreuter
minutely describes five varieties of the common tobacco,[236] which
were reciprocally crossed, and the offspring were intermediate in
character and as fertile as their parents: from this fact Kölreuter
inferred that they are really varieties; and no one, as far as I can
discover, seems to have doubted that such is the case. He also crossed
reciprocally these five varieties with _N. glutinosa_, and they yielded
very sterile hybrids; but those raised from the _var. perennis_,
whether used as the father or mother plant, were not so sterile as the
hybrids from the four other varieties.[237] So that the sexual {109}
capacity of this one variety has certainly been in some degree
modified, so as to approach in nature that of _N. glutinosa_.[238]

These facts with respect to plants show that in some few cases certain varieties have had their sexual powers so far modified, that they cross together less readily and yield less seed than other varieties of the same species. We shall presently see that the sexual functions of most animals and plants are eminently liable to be affected by the conditions of life to which they are exposed; and hereafter we shall briefly discuss the conjoint bearing of this and other facts on the difference in fertility between crossed varieties and crossed species.

_Domestication eliminates the tendency to Sterility which is general with Species when crossed._

This hypothesis was first propounded by Pallas,[239] and has been adopted by several authors. I can find hardly any direct facts in its support; but unfortunately no one has compared, in the case of either animals or plants, the fertility of anciently domesticated varieties, when crossed with a distinct species, with that of the wild parent-species when similarly crossed. No one has compared, for instance, the fertility of _Gallus bankiva_ and of the domesticated fowl, when crossed with a distinct species of Gallus or Phasianus; and the {110} experiment would in all cases be surrounded by many difficulties. Dureau de la Malle, who has so closely studied classical literature, states[240] that in the time of the Romans the common mule was produced with more difficulty than at the present day; but whether this statement may be trusted I know not. A much more important, though somewhat different, case is given by M. Groenland,[241] namely, that plants, known from their intermediate character and sterility to be hybrids between Ægilops and wheat, have perpetuated themselves under culture since 1857, _with a rapid but varying increase of fertility in each generation_. In the fourth generation the plants, still retaining their intermediate character, had become as fertile as common cultivated wheat.

The indirect evidence in favour of the Pallasian doctrine appears to me to be extremely strong. In the earlier chapters I have attempted to show that our various breeds of dogs are descended from several wild species; and this probably is the case with sheep. There can no longer be any doubt that the Zebu or humped Indian ox belongs to a distinct species from European cattle: the latter, moreover, are descended from two or three forms, which may be called either species or wild races, but which co-existed in a state of nature and kept distinct. We have good evidence that our domesticated pigs belong to at least two specific types, _S. scrofa_ and _Indica_, which probably lived together in a wild state in South-eastern Europe. Now, a widely-extended analogy leads to the belief that if these several allied species, in the wild state or when first reclaimed, had been crossed, they would have exhibited, both in their first unions and in their hybrid offspring, some degree of sterility. Nevertheless the several domesticated races descended from them are now all, as far as can be ascertained, perfectly fertile together. If this reasoning be trustworthy, and it is apparently sound, we must admit the Pallasian doctrine that long-continued domestication tends to eliminate that sterility which is natural to species when crossed in their aboriginal state.

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_On increased Fertility from Domestication and Cultivation._

Increased fertility from domestication, without any reference to crossing, may be here briefly considered. This subject bears indirectly on two or three points connected with the modification of organic beings. As Buffon long ago remarked,[242] domestic animals breed oftener in the year and produce more young at a birth than wild animals of the same species; they, also, sometimes breed at an earlier age. The case would hardly have deserved further notice, had not some authors lately attempted to show that fertility increases and decreases in an inverse ratio with the amount of food. This strange doctrine has apparently arisen from individual animals when supplied with an inordinate quantity of food, and from plants of many kinds when grown on excessively rich soil, as on a dunghill, becoming sterile; but to this latter point I shall have occasion presently to return. With hardly an exception, our domesticated animals, which have long been habituated to a regular and copious supply of food, without the labour of searching for it, are more fertile than the corresponding wild animals. It is notorious how frequently cats and dogs breed, and how many young they produce at a birth. The wild rabbit is said generally to breed four times yearly, and to produce from four to eight young; the tame rabbit breeds six or seven times yearly, and produces from four to eleven young. The ferret, though generally so closely confined, is more prolific than its supposed wild prototype. The wild sow is remarkably prolific, for she often breeds twice in the year, and produces from four to eight and sometimes even twelve young at a birth; but the domestic sow regularly breeds twice a year, and would breed oftener if permitted; and a sow that produces less than eight at a birth "is worth little, and the sooner she is fattened for the butcher the better." The amount of food affects the fertility even of the same individual: thus sheep, which on mountains never produce more than one lamb at a birth, when brought {112} down to lowland pastures frequently bear twins. This difference apparently is not due to the cold of the higher land, for sheep and other domestic animals are said to be extremely prolific in Lapland. Hard living, also, retards the period at which animals conceive; for it has been found disadvantageous in the northern islands of Scotland to allow cows to bear calves before they are four years old.[243]

Birds offer still better evidence of increased fertility from
domestication: the hen of the wild _Gallus bankiva_ lays from six to
ten eggs, a number which would be thought nothing of with the domestic
hen. The wild duck lays from five to ten eggs; the tame one in the
course of the year from eighty to one hundred. The wild grey-lag goose
lays from five to eight eggs; the tame from thirteen to eighteen, and
she lays a second time; as Mr. Dixon has remarked, "high-feeding, care,
and moderate warmth induce a habit of prolificacy which becomes in some
measure hereditary." Whether the semi-domesticated dovecot pigeon is
more fertile than the wild rock-pigeon _C. livia_, I know not; but the
more thoroughly domesticated breeds are nearly twice as fertile as
dovecots: the latter, however, when caged and highly fed, become
equally fertile with house pigeons. The peahen alone of domesticated
birds is rather more fertile, according to some accounts, when wild in
its native Indian home, than when domesticated in Europe and exposed to
our much colder climate.[244]

With respect to plants, no one would expect wheat to tiller more, and
each ear to produce more grain, in poor than in rich soil; or to get in
poor soil a heavy crop of peas or beans. Seeds vary so much in number
{113} that it is difficult to estimate them; but on comparing beds of
carrots saved for seed in a nursery garden with wild plants, the former
seemed to produce about twice as much seed. Cultivated cabbages yielded
thrice as many pods by measure as wild cabbages from the rocks of South
Wales. The excess of berries produced by the cultivated Asparagus in
comparison with the wild plant is enormous. No doubt many highly
cultivated plants, such as pears, pineapples, bananas, sugar-cane, &c.,
are nearly or quite sterile; and I am inclined to attribute this
sterility to excess of food and to other unnatural conditions; but to
this subject I shall presently recur.

In some cases, as with the pig, rabbit, &c., and with those plants which are valued for their seed, the direct selection of the more fertile individuals has probably much increased their fertility; and in all cases this may have occurred indirectly, from the better chance of the more numerous offspring produced by the more fertile individuals having survived. But with cats, ferrets, and dogs, and with plants like carrots, cabbages, and asparagus, which are not valued for their prolificacy, selection can have played only a subordinate part; and their increased fertility must be attributed to the more favourable conditions of life under which they have long existed.

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