Chapter XVII (1)

ON THE GOOD EFFECTS OF CROSSING, AND ON THE EVIL EFFECTS OF CLOSE INTERBREEDING.

DEFINITION OF CLOSE INTERBREEDING--AUGMENTATION OF MORBID
TENDENCIES--GENERAL EVIDENCE ON THE GOOD EFFECTS DERIVED FROM CROSSING,
AND ON THE EVIL EFFECTS FROM CLOSE INTERBREEDING--CATTLE, CLOSELY
INTERBRED; HALF-WILD CATTLE LONG KEPT IN THE SAME
PARKS--SHEEP--FALLOW-DEER--DOGS--RABBITS--PIGS--MAN, ORIGIN OF HIS
ABHORRENCE OF INCESTUOUS MARRIAGES--FOWLS--PIGEONS--HIVE-BEES--PLANTS,
GENERAL CONSIDERATIONS ON THE BENEFITS DERIVED FROM CROSSING--MELONS,
FRUIT-TREES, PEAS, CABBAGES, WHEAT, AND FOREST-TREES--ON THE INCREASED
SIZE OF HYBRID PLANTS, NOT EXCLUSIVELY DUE TO THEIR STERILITY--ON
CERTAIN PLANTS WHICH EITHER NORMALLY OR ABNORMALLY ARE SELF-IMPOTENT,
BUT ARE FERTILE, BOTH ON THE MALE AND FEMALE SIDE, WHEN CROSSED WITH
DISTINCT INDIVIDUALS EITHER OF THE SAME OR ANOTHER SPECIES--CONCLUSION.

The gain in constitutional vigour, derived from an occasional cross between individuals of the same variety, but belonging to distinct families, or between distinct varieties, has not been so largely or so frequently discussed, as have the evil effects of too close interbreeding. But the former point is the more important of the two, inasmuch as the evidence is more decisive. The evil results from close interbreeding are difficult to detect, for they accumulate slowly, and differ much in degree with different species; whilst the good effects which almost invariably follow a cross are from the first manifest. It should, however, be clearly understood that the advantage of close interbreeding, as far as the retention of character is concerned, is indisputable, and often outweighs the evil of a slight loss of constitutional vigour. In relation to the subject of domestication, the whole question is of some importance, as too close interbreeding interferes with the improvement of old races, and especially with the formation of new ones. It is important as indirectly bearing on Hybridism; and perhaps on the extinction of species, when any form has become so rare that only a few individuals {115} remain within a confined area. It bears in an important manner on the influence of free intercrossing, in obliterating individual differences, and thus giving uniformity of character to the individuals of the same race or species; for if additional vigour and fertility be thus gained, the crossed offspring will multiply and prevail, and the ultimate result will be far greater than otherwise would have occurred. Lastly, the question is of high interest, as bearing on mankind. Hence I shall discuss this subject at full length. As the facts which prove the evil effects of close interbreeding are more copious, though less decisive, than those on the good effects of crossing, I shall, under each group of beings, begin with the former.

There is no difficulty in defining what is meant by a cross; but this is by no means easy in regard to "breeding in and in" or "too close interbreeding," because, as we shall see, different species of animals are differently affected by the same degree of interbreeding. The pairing of a father and daughter, or mother and son, or brothers and sisters, if carried on during several generations, is the closest possible form of interbreeding. But some good judges, for instance Sir J. Sebright, believe that the pairing of a brother and sister is closer than that of parents and children; for when the father is matched with his daughter he crosses, as is said, with only half his own blood. The consequences of close interbreeding carried on for too long a time, are, as is generally believed, loss of size, constitutional vigour, and fertility, sometimes accompanied by a tendency to malformation. Manifest evil does not usually follow from pairing the nearest relations for two, three, or even four generations; but several causes interfere with our detecting the evil--such as the deterioration being very gradual, and the difficulty of distinguishing between such direct evil and the inevitable augmentation of any morbid tendencies which may be latent or apparent in the related parents. On the other hand, the benefit from a cross, even when there has not been any very close interbreeding, is almost invariably at once conspicuous. There is reason to believe, and this was the opinion of that most experienced observer Sir J. Sebright,[245] that the evil effects of close interbreeding may be checked by the related individuals {116} being separated during a few generations and exposed to different conditions of life.

That evil directly follows from any degree of close interbreeding has been denied by many persons; but rarely by any practical breeder; and never, as far as I know, by one who has largely bred animals which propagate their kind quickly. Many physiologists attribute the evil exclusively to the combination and consequent increase of morbid tendencies common to both parents: that this is an active source of mischief there can be no doubt. It is unfortunately too notorious that men and various domestic animals endowed with a wretched constitution, and with a strong hereditary disposition to disease, if not actually ill, are fully capable of procreating their kind. Close interbreeding, on the other hand, induces sterility; and this indicates something quite distinct from the augmentation of morbid tendencies common to both parents. The evidence immediately to be given convinces me that it is a great law of nature, that all organic beings profit from an occasional cross with individuals not closely related to them in blood; and that, on the other hand, long-continued close interbreeding is injurious.

Various general considerations have had much influence in leading me to this conclusion; but the reader will probably rely more on special facts and opinions. The authority of experienced observers, even when they do not advance the grounds of their belief, is of some little value. Now almost all men who have bred many kinds of animals and have written on the subject, such as Sir J. Sebright, Andrew Knight, &c.,[246] have expressed the strongest conviction on the impossibility of long-continued close interbreeding. Those who have compiled works on agriculture, and have associated much with breeders, such as the sagacious Youatt, Low, &c., have strongly declared their opinion to the same effect. Prosper Lucas, trusting largely to French authorities, has come to a similar conclusion. The distinguished German agriculturist Hermann von Nathusius, who has written the most able treatise on this subject which I have met with, concurs; and as I shall have to quote from {117} this treatise, I may state that Nathusius is not only intimately acquainted with works on agriculture in all languages, and knows the pedigrees of our British breeds better than most Englishmen, but has imported many of our improved animals, and is himself an experienced breeder.

Evidence of the evil effects of close interbreeding can most readily be acquired in the case of animals, such as fowls, pigeons, &c., which propagate quickly, and, from being kept in the same place, are exposed to the same conditions. Now I have inquired of very many breeders of these birds, and I have hitherto not met with a single man who was not thoroughly convinced that an occasional cross with another strain of the same sub-variety was absolutely necessary. Most breeders of highly-improved or fancy birds value their own strain, and are most unwilling, at the risk, in their opinion, of deterioration, to make a cross. The purchase of a first-rate bird of another strain is expensive, and exchanges are troublesome; yet all breeders, as far as I can hear, excepting those who keep large stocks at different places for the sake of crossing, are driven after a time to take this step.

Another general consideration which has had great influence on my mind is, that with all hermaphrodite animals and plants, which it might have been thought would have perpetually fertilised themselves, and thus have been subjected for long ages to the closest interbreeding, there is no single species, as far as I can discover, in which the structure ensures self-fertilisation. On the contrary, there are in a multitude of cases, as briefly stated in the fifteenth chapter, manifest adaptations which favour or inevitably lead to an occasional cross between one hermaphrodite and another of the same species; and these adaptive structures are utterly purposeless, as far as we can see, for any other end.

With _Cattle_ there can be no doubt that extremely close interbreeding
may be long carried on, advantageously with respect to external
characters and with no manifestly apparent evil as far as constitution
is concerned. The same remark is applicable to sheep. Whether these
animals have gradually been rendered less susceptible than others to
this evil, in order to permit them to live in herds,--a habit which
leads the old and vigorous males to expel all intruders, and in
consequence often to pair with their own daughters, I will not pretend
to decide. The case of Bakewell's Long-horns, which were closely
interbred for a long period, has often been {118} quoted; yet Youatt
says[247] the breed "had acquired a delicacy of constitution
inconsistent with common management," and "the propagation of the
species was not always certain." But the Shorthorns offer the most
striking case of close interbreeding; for instance, the famous bull
Favourite (who was himself the offspring of a half-brother and sister
from Foljambe) was matched with his own daughter, granddaughter, and
great-granddaughter; so that the produce of this last union, or the
great-great-granddaughter, had 15-16ths, or 93.75 per cent. of the
blood of Favourite in her veins. This cow was matched with the bull
Wellington, having 62.5 per cent. of Favourite blood in his veins, and
produced Clarissa; Clarissa was matched with the bull Lancaster, having
68.75 of the same blood, and she yielded valuable offspring.[248]
Nevertheless Collings, who reared these animals, and was a strong
advocate for close breeding, once crossed his stock with a Galloway,
and the cows from this cross realised the highest prices. Bates's herd
was esteemed the most celebrated in the world. For thirteen years he
bred most closely in and in; but during the next seventeen years,
though he had the most exalted notion of the value of his own stock, he
thrice infused fresh blood into his herd: it is said that he did this,
not to improve the form of his animals, but on account of their
lessened fertility. Mr. Bates's own view, as given by a celebrated
breeder,[249] was, that "to breed in and in from a bad stock was ruin
and devastation; yet that the practice may be safely followed within
certain limits when the parents so related are descended from
first-rate animals." We thus see that there has been extremely close
interbreeding with Shorthorns; but Nathusius, after the most careful
study of their pedigrees, says that he can find no instance of a
breeder who has strictly followed this practice during his whole life.
From this study and his own experience, he concludes that close
interbreeding is necessary to ennoble the stock; but that in effecting
this the greatest care is necessary, on account of the tendency to
infertility and weakness. It may be added, that another high
authority[250] asserts that many more calves are born cripples from
Shorthorns than from other and less closely interbred races of cattle.

Although by carefully selecting the best animals (as Nature effectually
does by the law of battle) close interbreeding may be long carried on
with cattle, yet the good effects of a cross between almost any two
breeds is at once shown by the greater size and vigour of the
offspring; as Mr. Spooner writes to me, "crossing distinct breeds
certainly improves cattle for the butcher." Such crossed animals are of
course of no value to the breeder; but they have been raised during
many years in several {119} parts of England to be slaughtered;[251]
and their merit is now so fully recognised, that at fat-cattle shows a
separate class has been formed for their reception. The best fat ox at
the great show at Islington in 1862 was a crossed animal.

The half-wild cattle, which have been kept in British parks probably
for 400 or 500 years, or even for a longer period, have been advanced
by Culley and others as a case of long-continued interbreeding within
the limits of the same herd without any consequent injury. With respect
to the cattle at Chillingham, the late Lord Tankerville owned that they
were bad breeders.[252] The agent, Mr. Hardy, estimates (in a letter to
me, dated May, 1861) that in the herd of about fifty the average number
annually slaughtered, killed by fighting, and dying, is about ten, or
one in five. As the herd is kept up to nearly the same average number,
the annual rate of increase must be likewise about one in five. The
bulls, I may add, engage in furious battles, of which battles the
present Lord Tankerville has given me a graphic description, so that
there will always be rigorous selection of the most vigorous males. I
procured in 1855 from Mr. D. Gardner, agent to the Duke of Hamilton,
the following account of the wild cattle kept in the Duke's park in
Lanarkshire, which is about 200 acres in extent. The number of cattle
varies from sixty-five to eighty; and the number annually killed (I
presume by all causes) is from eight to ten; so that the annual rate of
increase can hardly be more than one in six. Now in South America,
where the herds are half-wild, and therefore offer a nearly fair
standard of comparison, according to Azara the natural increase of the
cattle on an estancia is from one-third to one-fourth of the total
number, or one in between three and four; and this, no doubt, applies
exclusively to adult animals fit for consumption. Hence the half-wild
British cattle which have long interbred within the limits of the same
herd are relatively far less fertile. Although in an unenclosed country
like Paraguay there must be some crossing between the different herds,
yet even there the inhabitants believe that the occasional introduction
of animals from distant localities is necessary to prevent
"degeneration in size and diminution of fertility."[253] The decrease
in size from ancient times in the Chillingham and Hamilton cattle must
have been prodigious, for Professor Rütimeyer has shown that they are
almost certainly the descendants of the gigantic _Bos primigenius_. No
doubt this decrease in size may be largely attributed to less
favourable conditions of life; yet animals roaming over large parks,
and fed during severe winters, can hardly be considered as placed under
very unfavourable conditions.

With _Sheep_ there has often been long-continued interbreeding within
the limits of the same flock; but whether the nearest relations have
been matched so frequently as in the case of Shorthorn cattle, I do not
know. The Messrs. Brown during fifty years have never infused fresh
blood into their excellent flock of Leicesters. Since 1810 Mr. Barford
has acted on the same principle with the Foscote flock. He asserts that
half a century {120} of experience has convinced him that when two
nearly related animals are quite sound in constitution, in-and-in
breeding does not induce degeneracy; but he adds that he "does not
pride himself on breeding from the nearest affinities." In France the
Naz flock has been bred for sixty years without the introduction of a
single strange ram.[254] Nevertheless, most great breeders of sheep
have protested against close interbreeding prolonged for too great a
length of time.[255] The most celebrated of recent breeders, Jonas
Webb, kept five separate families to work on, thus "retaining the
requisite distance of relationship between the sexes."[256]

Although by the aid of careful selection the near interbreeding of
sheep may be long continued without any manifest evil, yet it has often
been the practice with farmers to cross distinct breeds to obtain
animals for the butcher, which plainly shows that good is derived from
this practice. Mr. Spooner sums up his excellent Essay on Crossing by
asserting that there is a direct pecuniary advantage in judicious
cross-breeding, especially when the male is larger than the female. A
former celebrated breeder, Lord Somerville, distinctly states that his
half-breeds from Ryelands and Spanish sheep were larger animals than
either the pure Ryelands or pure Spanish sheep.[257]

As some of our British parks are ancient, it occurred to me that there
must have been long-continued close interbreeding with the fallow deer
(_Cervus dama_) kept in them; but on inquiry I find that it is a common
practice to infuse new blood by procuring bucks from other parks. Mr.
Shirley,[258] who has carefully studied the management of deer, admits
that in some parks there has been no admixture of foreign blood from a
time beyond the memory of man. But he concludes "that in the end the
constant breeding in-and-in is sure to tell to the disadvantage of the
whole herd, though it may take a very long time to prove it; moreover,
when we find, as is very constantly the case, that the introduction of
fresh blood has been of the very greatest use to deer, both by
improving their size and appearance, and particularly by being of
service in removing the taint of 'rickback,' if not of other diseases,
to which deer are sometimes subject when the blood has not been
changed, there can, I think, be no doubt but that a judicious cross
with a good stock is of the greatest consequence, and is indeed
essential, sooner or later, to the prosperity of every well-ordered
park."

Mr. Meynell's famous foxhounds have been adduced, as showing that no
ill effects follow from close interbreeding; and Sir J. Sebright
ascertained from him that he frequently bred from father and daughter,
mother and {121} son, and sometimes even from brothers and sisters. Sir
J. Sebright, however, declares,[259] that by breeding _in-and-in_, by
which he means matching brothers and sisters, he has actually seen
strong spaniels become weak and diminutive lapdogs. The Rev. W. D. Fox
has communicated to me the case of a small lot of bloodhounds, long
kept in the same family, which had become very bad breeders, and nearly
all had a bony enlargement in the tail. A single cross with a distinct
strain of bloodhounds restored their fertility, and drove away the
tendency to malformation in the tail. I have heard the particulars of
another case with bloodhounds, in which the female had to be held to
the male. Considering how rapid is the natural increase of the dog, it
is difficult to understand the high price of most highly improved
breeds, which almost implies long-continued close interbreeding, except
on the belief that this process lessens fertility and increases
liability to distemper and other diseases. A high authority, Mr.
Scrope, attributes the rarity and deterioration in size of the Scotch
deerhound (the few individuals now existing throughout the country
being all related) in large part to close interbreeding.

With all highly-bred animals there is more or less difficulty in
getting them to procreate quickly, and all suffer much from delicacy of
constitution; but I do not pretend that these effects ought to be
wholly attributed to close interbreeding. A great judge of rabbits[260]
says, "the long-eared does are often too highly bred or forced in their
youth to be of much value as breeders, often turning out barren or bad
mothers." Again: "Very long-eared bucks will also sometimes prove
barren." These highly-bred rabbits often desert their young, so that it
is necessary to have nurse-rabbits.

With _Pigs_ there is more unanimity amongst breeders on the evil
effects of close interbreeding than, perhaps, with any other large
animal. Mr. Druce, a great and successful breeder of the Improved
Oxfordshires (a crossed race), writes, "without a change of boars of a
different tribe, but of the same breed, constitution cannot be
preserved." Mr. Fisher Hobbs, the raiser of the celebrated Improved
Essex breed, divided his stock into three separate families, by which
means he maintained the breed for more than twenty years, "by judicious
selection from the _three distinct families_."[261] Lord Western was
the first importer of a Neapolitan boar and sow. "From this pair he
bred in-and-in, until the breed was in danger of becoming extinct, a
sure result (as Mr. Sidney remarks) of in-and-in breeding." Lord
Western then crossed his Neapolitan pigs with the old Essex, and made
the first great step towards the Improved Essex breed. Here is a more
interesting case. Mr. J. Wright, well known as a breeder, crossed[262]
the same boar with the daughter, granddaughter, and
great-granddaughter, and so on for seven generations. The result was,
that in many instances the offspring failed to breed; in others they
produced few that lived; and of the latter many were idiotic, without
sense {122} even to suck, and when attempting to move could not walk
straight. Now it deserves especial notice, that the two last sows
produced by this long course of interbreeding were sent to other boars,
and they bore several litters of healthy pigs. The best sow in external
appearance produced during the whole seven generations was one in the
last stage of descent; but the litter consisted of this one sow. She
would not breed to her sire, yet bred at the first trial to a stranger
in blood. So that, in Mr. Wright's case, long-continued and extremely
close interbreeding did not affect the external form or merit of the
young; but with many of them the general constitution and mental
powers, and especially the reproductive functions, were seriously
affected.

Nathusius gives[263] an analogous and even more striking case: he
imported from England a pregnant sow of the large Yorkshire breed, and
bred the product closely in-and-in for three generations: the result
was unfavourable, as the young were weak in constitution, with impaired
fertility. One of the latest sows, which he esteemed a good animal,
produced, when paired with her own uncle (who was known to be
productive with sows of other breeds), a litter of six, and a second
time a litter of only five weak young pigs. He then paired this sow
with a boar of a small black breed, which he had likewise imported from
England, and which boar, when matched with sows of his own breed,
produced from seven to nine young: now, the sow of the large breed,
which was so unproductive when paired with her own uncle, yielded to
the small black boar, in the first litter twenty-one, and in the second
litter eighteen young pigs; so that in one year she produced
thirty-nine fine young animals!

As in the case of several other animals already mentioned, even when no
injury is perceptible from moderately close interbreeding, yet, to
quote the words of Mr. Coate, a most successful breeder (who five times
won the annual gold medal of the Smithfield Club Show for the best pen
of pigs), "Crosses answer well for profit to the farmer, as you get
more constitution and quicker growth; but for me, who sell a great
number of pigs for breeding purposes, I find it will not do, as it
requires many years to get anything like purity of blood again."[264]

Before passing on to Birds, I ought to refer to man, though I am unwilling to enter on this subject, as it is surrounded by natural prejudices. It has moreover been discussed by various authors under many points of view.[265] Mr. Tylor[266] has shown {123} that with widely different races, in the most distant quarters of the world, marriages between relations--even between distant relations--have been strictly prohibited. A few exceptional cases can be specified, especially with royal families; and these have been enlarged on in a learned article[267] by Mr. W. Adam, and formerly in 1828 by Hofacker. Mr. Tylor is inclined to believe that the almost universal prohibition of closely-related marriages has arisen from their evil effects having been observed, and he ingeniously explains some apparent anomalies in the prohibition not extending equally to the relations on both the male and female side. He admits, however, that other causes, such as the extension of friendly alliances, may have come into play. Mr. W. Adam, on the other hand, concludes that related marriages are prohibited and viewed with repugnance from the confusion which would thus arise in the descent of property, and from other still more recondite reasons; but I cannot accept this view, seeing that the savages of Australia and South America,[268] who have no property to bequeath or fine moral feelings to confuse, hold the crime of incest in abhorrence.

It would be interesting to know, if it could be ascertained, as throwing light on this question with respect to man, what occurs with the higher anthropomorphous apes--whether the young males and females soon wander away from their parents, or whether the old males become jealous of their sons and expel them, or whether any inherited instinctive feeling, from being beneficial, has been generated, leading the young males and females of the same family to prefer pairing with distinct families, and to dislike pairing with each other. A considerable body of evidence has already been advanced, showing that the offspring from parents which are not related are more vigorous and fertile than those from parents which are closely related; hence any slight feeling, arising from the sexual excitement of novelty or other cause, which led to the former rather than to the latter unions, would be augmented through natural selection, and thus might become instinctive; for those individuals which had an innate preference of this kind would increase in number. It seems more probable, that degraded savages should {124} thus unconsciously have acquired their dislike and even abhorrence of incestuous marriages, rather than that they should have discovered by reasoning and observation the evil results. The abhorrence occasionally failing is no valid argument against the feeling being instinctive, for any instinct may occasionally fail or become vitiated, as sometimes occurs with parental love and the social sympathies. In the case of man, the question whether evil follows from close interbreeding will probably never be answered by direct evidence, as he propagates his kind so slowly and cannot be subjected to experiment; but the almost universal practice of all races at all times of avoiding closely-related marriages is an argument of considerable weight; and whatever conclusion we arrive at in regard to the higher animals may be safely extended to man.

Turning now to Birds: in the case of the _Fowl_ a whole array of
authorities could be given against too close interbreeding. Sir J.
Sebright positively asserts that he made many trials, and that his
fowls, when thus treated, became long in the legs, small in the body,
and bad breeders.[269] He produced the famous Sebright Bantams by
complicated crosses, and by breeding in-and-in; and since his time
there has been much close interbreeding with these Bantams; and they
are now notoriously bad breeders. I have seen Silver Bantams, directly
descended from his stock, which had become almost as barren as hybrids;
for not a single chicken had been that year hatched from two full nests
of eggs. Mr. Hewitt says that with these Bantams the sterility of the
male stands, with rare exceptions, in the closest relation with their
loss of certain secondary male characters: he adds, "I have noticed, as
a general rule, that even the slightest deviation from feminine
character in the tail of the male Sebright--say the elongation by only
half an inch of the two principal tail-feathers--brings with it
improved probability of increased fertility."[270]

Mr. Wright states[271] that Mr. Clark, "whose fighting-cocks were so
notorious, continued to breed from his own kind till they lost their
disposition to fight, but stood to be cut up without making any
resistance, and were so reduced in size as to be under those weights
required for the best prizes; but on obtaining a cross from Mr.
Leighton, they again resumed their former courage and weight." It
should be borne in mind that game-cocks before they fought were always
weighed, so that nothing was left to the imagination about any
reduction or increase of {125} weight. Mr. Clark does not seem to have
bred from brothers and sisters, which is the most injurious kind of
union; and he found, after repeated trials, that there was a greater
reduction in weight in the young from a father paired with his
daughter, than from a mother with her son. I may add that Mr. Eyton, of
Eyton, the well-known ornithologist, who is a large breeder of Grey
Dorkings, informs me that they certainly diminish in size, and become
less prolific, unless a cross with another strain is occasionally
obtained. So it is with Malays, according to Mr. Hewitt, as far as size
is concerned.[272]

An experienced writer[273] remarks that the same amateur, as is well
known, seldom long maintains the superiority of his birds; and this, he
adds, undoubtedly is due to all his stock "being of the same blood;"
hence it is indispensable that he should occasionally procure a bird of
another strain. But this is not necessary with those who keep a stock
of fowls at different stations. Thus, Mr. Ballance, who has bred Malays
for thirty years, and has won more prizes with these birds than any
other fancier in England, says that breeding in-and-in does not
necessarily cause deterioration; "but all depends upon how this is
managed." "My plan has been to keep about five or six distinct runs,
and to rear about two hundred or three hundred chickens each year, and
select the best birds from each run for crossing. I thus secure
sufficient crossing to prevent deterioration."[274]

We thus see that there is almost complete unanimity with
poultry-breeders that, when fowls are kept at the same place, evil
quickly follows from interbreeding carried on to an extent which would
be disregarded in the case of most quadrupeds. On the other hand, it is
a generally received opinion that cross-bred chickens are the hardiest
and most easily reared.[275] Mr. Tegetmeier, who has carefully attended
to poultry of all breeds, says[276] that Dorking hens, allowed to run
with Houdan or Crevecoeur cocks, "produce in the early spring chickens
that for size, hardihood, early maturity, and fitness for the market,
surpass those of any pure breed that we have ever raised." Mr. Hewitt
gives it as a general rule with fowls, that crossing the breed
increases their size. He makes this remark after stating that hybrids
from the pheasant and fowl are considerably larger than either
progenitor: so again, hybrids from the male golden pheasant and hen
common pheasant "are of far larger size than either parent-bird."[277]
To this subject of the increased size of hybrids I shall presently
return.

With _Pigeons_, breeders are unanimous, as previously stated, that it
is absolutely indispensable, notwithstanding the trouble and expense
thus caused, occasionally to cross their much-prized birds with
individuals of another strain, but belonging, of course, to the same
variety. It deserves {126} notice that, when large size is one of the
desired characters, as with pouters,[278] the evil effects of close
interbreeding are much sooner perceived than when small birds, such as
short-faced tumblers, are valued. The extreme delicacy of the high
fancy breeds, such as these tumblers and improved English carriers, is
remarkable; they are liable to many diseases, and often die in the egg
or during the first moult; and their eggs have generally to be hatched
under foster-mothers. Although these highly-prized birds have
invariably been subjected to much close interbreeding, yet their
extreme delicacy of constitution cannot perhaps be thus fully
explained. Mr. Yarrell informed me that Sir J. Sebright continued
closely interbreeding some owl-pigeons, until from their extreme
sterility he as nearly as possible lost the whole family. Mr.
Brent[279] tried to raise a breed of trumpeters, by crossing a common
pigeon, and recrossing the daughter, granddaughter,
great-granddaughter, and great-great-granddaughter, with the same male
trumpeter, until he obtained a bird with 15/16ths of trumpeter's blood;
but then the experiment failed, for "breeding so close stopped
reproduction." The experienced Neumeister[280] also asserts that the
offspring from dovecotes and various other breeds are "generally very
fertile and hardy birds:" so again, MM. Boitard and Corbié,[281] after
forty-five years' experience, recommend persons to cross their breeds
for amusement; for, if they fail to make interesting birds, they will
succeed under an economical point of view, "as it is found that
mongrels are more fertile than pigeons of pure race."

I will refer only to one other animal, namely, the Hive-bee, because a
distinguished entomologist has advanced this as a case of inevitable
close interbreeding. As the hive is tenanted by a single female, it
might have been thought that her male and female offspring would always
have bred together, more especially as bees of different hives are
hostile to each other; a strange worker being almost always attacked
when trying to enter another hive. But Mr. Tegetmeier has shown[282]
that this instinct does not apply to drones, which are permitted to
enter any hive; so that there is no _à priori_ improbability of a queen
receiving a foreign drone. The fact of the union invariably and
necessarily taking place on the wing, during the queen's nuptial
flight, seems to be a special provision against continued
interbreeding. However this may be, experience has shown, since the
introduction of the yellow-banded Ligurian race into Germany and
England, that bees freely cross: Mr. Woodbury, who introduced Ligurian
bees into Devonshire, found during a single season that three stocks,
at distances of from one to two miles from his hives, were crossed by
his drones. In one case the Ligurian drones must have flown over the
city of Exeter, and over several intermediate hives. On another
occasion several common black queens were crossed by Ligurian drones at
a distance of from one to three and a half miles.[283]

{127}

_Plants._

When a single plant of a new species is introduced into any country, if
propagated by seed, many individuals will soon be raised, so that if
the proper insects be present there will be crossing. With
newly-introduced trees or other plants not propagated by seed we are
not here concerned. With old-established plants it is an almost
universal practice occasionally to make exchanges of seed, by which
means individuals which have been exposed to different conditions of
life,--and this, as we have seen, diminishes the evil from close
interbreeding,--will occasionally be introduced into each district.

Experiments have not been tried on the effects of fertilising flowers
with their own pollen during _several_ generations. But we shall
presently see that certain plants, either normally or abnormally, are
more or less sterile, even in the first generation, when fertilised by
their own pollen. Although nothing is directly known on the evil
effects of long-continued close interbreeding with plants, the converse
proposition that great good is derived from crossing is well
established.

With respect to the crossing of individuals belonging to the same
sub-variety, Gärtner, whose accuracy and experience exceeded that of
all other hybridisers, states[284] that he has many times observed good
effects from this step, especially with exotic genera, of which the
fertility is somewhat impaired, such as Passiflora, Lobelia, and
Fuchsia. Herbert also says,[285] "I am inclined to think that I have
derived advantage from impregnating the flower from which I wished to
obtain seed with pollen from another individual of the same variety, or
at least from another flower, rather than with its own." Again,
Professor Lecoq asserts that he has ascertained that crossed offspring
are more vigorous and robust than their parents.[286]

General statements of this kind, however, can seldom be fully trusted;
consequently I have begun a series of experiments, which, if they
continue to give the same results as hitherto, will for ever settle the
question of the good effects of crossing two distinct plants of the
same variety, and of the evil effects of self-fertilisation. A clear
light will thus also be thrown on the fact that flowers are invariably
constructed so as to permit, or favour, or necessitate the union of two
individuals. We shall clearly understand why monoecious and
dioecious,--why dimorphic and trimorphic plants exist, and many other
such cases. The plan which I have followed in my experiments is to grow
plants in the same pot, or in pots of the same size, or close together
in the open ground; to carefully exclude insects; and then to fertilise
some of the flowers with pollen from the same flower, and others on the
same plant with pollen from a distinct but adjoining plant. In many,
but not all, of these experiments, the crossed plants yielded much more
seed than the self-fertilised plants; and I have never seen the {128}
reversed case. The self-fertilised and crossed seeds thus obtained were
allowed to germinate in the same glass vessel on damp sand; and as the
seeds successively germinated, they were planted in pairs on opposite
sides of the same pot, with a superficial partition between them, and
were placed so as to be equally exposed to the light. In other cases
the self-fertilised and crossed seeds were simply sown on opposite
sides of the same small pot. I have, in short, followed different
plans, but in every case have taken all the precautions which I could
think of, so that the two lots should be equally favoured. Now, I have
carefully observed the growth of plants raised from crossed and
self-fertilised seed, from their germination to maturity, in species of
the following genera, namely, Brassica, Lathyrus, Lupinus, Lobelia,
Lactuca, Dianthus, Myosotis, Petunia, Linaria, Calceolaria, Mimulus,
and Ipomoea, and the difference in their powers of growth, and of
withstanding in certain cases unfavourable conditions, was most
manifest and strongly marked. It is of importance that the two lots of
seed should be sown or planted on opposite sides of the same pot, so
that the seedlings may struggle against each other; for if sown
separately in ample and good soil, there is often but little difference
in their growth.

I will briefly describe the two most striking cases as yet observed by
me. Six crossed and six self-fertilised seeds of _Ipomoea purpurea_,
from plants treated in the manner above described, were planted as soon
as they had germinated, in pairs on opposite sides of two pots, and
rods of equal thickness were given them to twine up. Five of the
crossed plants grew from the first more quickly than the opposed
self-fertilised plants; the sixth, however, was weakly and was for a
time beaten, but at last its sounder constitution prevailed and it shot
ahead of its antagonist. As soon as each crossed plant reached the top
of its seven-foot rod its fellow was measured, and the result was that,
when the crossed plants were seven feet high, the self-fertilised had
attained the average height of only five feet four and a half inches.
The crossed plants flowered a little before, and more profusely than
the self-fertilised plants. On opposite sides of another _small_ pot a
large number of crossed and self-fertilised seeds were sown, so that
they had to struggle for bare existence; a single rod was given to each
lot: here again the crossed plants showed from the first their
advantage; they never quite reached the summit of the seven-foot rod,
but relatively to the self-fertilised plants their average height was
as seven feet to five feet two inches. The experiment was repeated in
the two following generations with plants raised from the
self-fertilised and crossed plants, treated in exactly the same manner,
and with nearly the same result. In the second generation, the crossed
plants, which were again crossed, produced 121 seed-capsules, whilst
the self-fertilised plants, again self-fertilised, produced only 84
capsules.

Some flowers of the _Mimulus luteus_ were fertilised with their own
pollen, and others were crossed with pollen from distinct plants
growing in the same pot. The seeds after germinating were thickly
planted on opposite sides of a pot. The seedlings were at first equal
in height; but when the young crossed plants were exactly half an inch,
the {129} self-fertilised plants were only a quarter of an inch high.
But this inequality did not continue, for, when the crossed plants were
four and a half inches high, the self-fertilised were three inches; and
they retained the same relative difference till their growth was
complete. The crossed plants looked far more vigorous than the
uncrossed, and flowered before them; they produced also a far greater
number of flowers, which yielded capsules (judging, however, from only
a few) containing more seeds. As in the former case, the experiment was
repeated in the same manner during the next two generations, and with
exactly the same result. Had I not watched these plants of the Mimulus
and Ipomoea during their whole growth, I could not have believed it
possible, that a difference apparently so slight, as that of the pollen
being taken from the same flower, and from a distinct plant growing in
the same small pot, could have made so wonderful a difference in the
growth and vigour of the plants thus produced. This, under a
physiological point of view, is a most remarkable phenomenon.

* * * * *

With respect to the benefit derived from crossing distinct varieties,
plenty of evidence has been published. Sageret[287] repeatedly speaks
in strong terms of the vigour of melons raised by crossing different
varieties, and adds that they are more easily fertilised than common
melons, and produce numerous good seed. Here follows the evidence of an
English gardener:[288] "I have this summer met with better success in
my cultivation of melons, in an unprotected state, from the seeds of
hybrids (_i.e._ mongrels) obtained by cross impregnation, than with old
varieties. The offspring of three different hybridisations (one more
especially, of which the parents were the two most dissimilar varieties
I could select) each yielded more ample and finer produce than any one
of between twenty and thirty established varieties."

Andrew Knight[289] believed that his seedlings from crossed varieties
of the apple exhibited increased vigour and luxuriance; and M.
Chevreul[290] alludes to the extreme vigour of some of the crossed
fruit-trees raised by Sageret.

By crossing reciprocally the tallest and shortest peas, Knight[291]
says, "I had in this experiment a striking instance of the stimulative
effects of crossing the breeds; for the smallest variety, whose height
rarely exceeded two feet, was increased to six feet; whilst the height
of the large and luxuriant kind was very little diminished." Mr. Laxton
gave me seed-peas produced from crosses between four distinct kinds;
and the plants thus raised were extraordinarily vigorous, being in each
case from one to two or three feet taller than the parent-forms growing
close alongside them.

{130}

Wiegmann[292] made many crosses between several varieties of cabbage;
and he speaks with astonishment of the vigour and height of the
mongrels, which excited the amazement of all the gardeners who beheld
them. Mr. Chaundy raised a great number of mongrels by planting
together six distinct varieties of cabbage. These mongrels displayed an
infinite diversity of character; "But the most remarkable circumstance
was, that, while all the other cabbages and borecoles in the nursery
were destroyed by a severe winter, these hybrids were little injured,
and supplied the kitchen when there was no other cabbage to be had."

Mr. Maund exhibited before the Royal Agricultural Society[293]
specimens of crossed wheat, together with their parent varieties; and
the editor states that they were intermediate in character, "united
with that greater vigour of growth, which it appears, in the vegetable
as in the animal world, is the result of a first cross." Knight also
crossed several varieties of wheat,[294] and he says "that in the years
1795 and 1796, when almost the whole crop of corn in the island was
blighted, the varieties thus obtained, and these only, escaped in this
neighbourhood, though sown in several different soils and situations."

Here is a remarkable case: M. Clotzsch[295] crossed _Pinus sylvestris_
and _nigricans_, _Quercus robur_ and _pedunculata, Alnus glutinosa_ and
_incana_, _Ulmus campestris_ and _effusa_; and the cross-fertilised
seeds, as well as seeds of the pure parent-trees, were all sown at the
same time and in the same place. The result was, that after an interval
of eight years, the hybrids were one-third taller than the pure trees!

* * * * *

The facts above given refer to undoubted varieties, excepting the trees
crossed by Clotzsch, which are ranked by various botanists as
strongly-marked races, sub-species, or species. That true hybrids
raised from entirely distinct species, though they lose in fertility,
often gain in size and constitutional vigour, is certain. It would be
superfluous to quote any facts; for all experimenters, Kölreuter,
Gärtner, Herbert, Sageret, Lecoq, and Naudin, have been struck with the
wonderful vigour, height, size, tenacity of life, precocity, and
hardiness of their hybrid productions. Gärtner[296] sums up his
conviction on this head in the strongest terms. Kölreuter[297] gives
numerous precise measurements of the weight and height of his hybrids
in comparison with measurements of both parent-forms; and speaks with
astonishment of their "_statura portentosa_," their "_ambitus
vastissimus ac altitudo valde conspicua_." Some exceptions to the rule
in the case of very sterile hybrids have, however, been noticed by
Gärtner and {131} Herbert; but the most striking exceptions are given
by Max Wichura,[298] who found that hybrid willows were generally
tender in constitution, dwarf, and short-lived.

Kölreuter explains the vast increase in the size of the roots, stems,
&c., of his hybrids, as the result of a sort of compensation due to
their sterility, in the same way as many emasculated animals are larger
than the perfect males. This view seems at first sight extremely
probable, and has been accepted by various authors;[299] but
Gärtner[300] has well remarked that there is much difficulty in fully
admitting it; for with many hybrids there is no parallelism between the
degree of their sterility and their increased size and vigour. The most
striking instances of luxuriant growth have been observed with hybrids
which were not sterile in any extreme degree. In the genus Mirabilis,
certain hybrids are unusually fertile, and their extraordinary
luxuriance of growth, together with their enormous roots,[301] have
been transmitted to their progeny. The increased size of the hybrids
produced between the fowl and pheasant, and between the distinct
species of pheasants, has been already noticed. The result in all cases
is probably in part due to the saving of nutriment and vital force
through the sexual organs not acting, or acting imperfectly, but more
especially to the general law of good being derived from a cross. For
it deserves especial attention that mongrel animals and plants, which
are so far from being sterile that their fertility is often actually
augmented, have, as previously shown, their size, hardiness, and
constitutional vigour generally increased. It is not a little
remarkable that an accession of vigour and size should thus arise under
the opposite contingencies of increased and diminished fertility.

It is a perfectly well ascertained fact[302] that hybrids will
invariably breed more readily with either pure parent, and not rarely
with a distinct species, than with each other. Herbert is inclined to
explain even this fact by the advantage derived from a cross; but
Gärtner more justly accounts for it by the pollen of the hybrid, and
probably its ovules, being in some degree vitiated, whereas the pollen
and ovules of both pure parents and of any third species are sound.
Nevertheless there are some well-ascertained and remarkable facts,
which, as we shall immediately see, show that the act of crossing in
itself undoubtedly tends to increase or re-establish the fertility of
hybrids.

_On certain Hermaphrodite Plants which, either normally or abnormally, require to be fertilised by pollen from a distinct individual or species._

The facts now to be given differ from those hitherto detailed, as the self-sterility does not here result from long-continued, {132} close interbreeding. These facts are, however, connected with our present subject, because a cross with a distinct individual is shown to be either necessary or advantageous. Dimorphic and trimorphic plants, though they are hermaphrodites, must be reciprocally crossed, one set of forms by the other, in order to be fully fertile, and in some cases to be fertile in any degree. But I should not have noticed these plants, had it not been for the following cases given by Dr. Hildebrand:[303]--

_Primula sinensis_ is a reciprocally dimorphic species: Dr. Hildebrand
fertilised twenty-eight flowers of both forms, each by pollen of the
other form, and obtained the full number of capsules containing on an
average 42.7 seed per capsule; here we have complete and normal
fertility. He then fertilised forty-two flowers of both forms with
pollen of the same form, but taken from a distinct plant, and all
produced capsules containing on an average only 19.6 seed. Lastly, and
here we come to our more immediate point, he fertilised forty-eight
flowers of both forms with pollen of the same form, taken from the same
flower, and now he obtained only thirty-two capsules, and these
contained on an average 18.6 seed, or one less per capsule than in the
former case. So that, with these illegitimate unions, the act of
impregnation is less assured, and the fertility slightly less, when the
pollen and ovules belong to the same flower, than when belonging to two
distinct individuals of the same form. Dr. Hildebrand has recently made
analogous experiments on the long-styled form of _Oxalis rosea_, with
the same result.[304]

It has recently been discovered that certain plants, whilst growing in their native country under natural conditions, cannot be fertilised with pollen from the same plant. They are sometimes so utterly self-impotent, that, though they can readily be fertilised by the pollen of a distinct species or even distinct genus, yet, wonderful as the fact is, they never produce a single seed by their own pollen. In some cases, moreover, the plant's own pollen and stigma mutually act on each other in a deleterious manner. Most of the facts to be given relate to Orchids, but I will commence with a plant belonging to a widely different family.

Sixty-three flowers of _Corydalis cava_, borne on distinct plants, were
fertilised by Dr. Hildebrand[305] with pollen from other plants of the
same species; and fifty-eight capsules were obtained, including on an
average {133} 4.5 seed in each. He then fertilised sixteen flowers
produced by the same raceme, one with another, but obtained only three
capsules, one of which alone contained any good seeds, namely, two in
number. Lastly, he fertilised twenty-seven flowers, each with its own
pollen; he left also fifty-seven flowers to be spontaneously
fertilised, and this would certainly have ensued if it had been
possible, for the anthers not only touch the stigma, but the
pollen-tubes were seen by Dr. Hildebrand to penetrate it; nevertheless
these eighty-four flowers did not produce a single seed-capsule! This
whole case is highly instructive, as it shows how widely different the
action of the same pollen is, according as it is placed on the stigma
of the same flower, or on that of another flower on the same raceme, or
on that of a distinct plant.

With exotic Orchids several analogous cases have been observed, chiefly
by Mr. John Scott.[306] _Oncidium sphacelatum_ has effective pollen,
for with it Mr. Scott fertilised two distinct species; its ovules are
likewise capable of impregnation, for they were readily fertilised by
the pollen of _O. divaricatum_; nevertheless, between one and two
hundred flowers fertilised by their own pollen did not produce a single
capsule, though the stigmas were penetrated by the pollen-tubes. Mr.
Robinson Munro, of the Royal Botanic Gardens of Edinburgh, also informs
me (1864) that a hundred and twenty flowers of this same species were
fertilised by him with their own pollen, and did not produce a capsule,
but eight flowers fertilised by the pollen of _O. divaricatum_ produced
four fine capsules: again, between two and three hundred flowers of _O.
divaricatum_, fertilised by their own pollen, did not set a capsule,
but twelve flowers fertilised by _O. flexuosum_ produced eight fine
capsules: so that here we have three utterly self-impotent species,
with their male and female organs perfect, as shown by their mutual
fertilisation. In these cases fertilisation was effected only by the
aid of a distinct species. But, as we shall presently see, distinct
plants, raised from seed, of _Oncidium flexuosum_, and probably of the
other species, would have been perfectly capable of fertilising each
other, for this is the natural process. Again, Mr. Scott found that the
pollen of a plant of _O. microchilum_ was good, for with it he
fertilised two distinct species; he found its ovules good, for they
could be fertilised by the pollen of one of these species, and by the
pollen of a distinct plant of _O. microchilum_; but they could not be
fertilised by pollen of the same plant, though the pollen-tubes
penetrated the stigma. An analogous case has been recorded by M.
Rivière,[307] with two plants of _O. Cavendishianum_, which were both
self-sterile, but reciprocally fertilised each other. All these cases
refer to the genus Oncidium, but Mr. Scott found that _Maxillaria
atro-rubens_ was "totally insusceptible of fertilisation with its own
pollen," but fertilised, and was fertilised by, a widely distinct
species, viz. _M. squalens_.