CHAPTER 2.XVII.

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

INTERBREEDING.

DEFINITION OF CLOSE INTERBREEDING.

AUGMENTATION OF MORBID TENDENCIES.

GENERAL EVIDENCE OF 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. It is important as indirectly bearing on Hybridism; and possibly on

the extinction of species, when any form has become so rare that only a few

individuals 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. I shall therefore 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 much 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 good reason to believe, and this was the opinion of that

most experienced observer Sir J. Sebright (17/1. 'The Art of Improving the

Breed, etc.' 1809 page 16.), that the evil effects of close interbreeding may

be checked or quite prevented by the related individuals being separated for a

few generations and exposed to different conditions of life. This conclusion

is now held by many breeders; for instance Mr. Carr (17/2. 'The History of the

Rise and Progress of the Killerby, etc. Herds' page 41.) remarks, it is a

well-known "fact that a change of soil and climate effects perhaps almost as

great a change in the constitution as would result from an infusion of fresh

blood." I hope to show in a future work that consanguinity by itself counts

for nothing, but acts solely from related organisms generally having a similar

constitution, and having been exposed in most cases to similar conditions.

That any evil directly follows from the closest 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; and 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, often 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, etc. (17/3. For Andrew Knight see A. Walker

on 'Intermarriage' 1838 page 227. Sir J. Sebright 'Treatise' has just been

quoted.), 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, etc., 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 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, etc., 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 been thus subjected

for long ages to the closest interbreeding, there is not a 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 manifest evil as far as constitution is concerned. The case of Bakewell's

Longhorns, which were closely interbred for a long period, has often been

quoted; yet Youatt says (17/4. 'Cattle' page 199.) 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. (17/5. I give this on the authority of Nathusius 'Ueber

Shorthorn Rindvieh' 1857 s. 71, see also 'Gardeners Chronicle' 1860 page 270.

But Mr. J. Storer, a large breeder of cattle, informs me that the parentage of

Clarissa is not well authenticated. In the first volume of the 'Herd Book' she

was entered as having six descents from Favourite, "which was a palpable

mistake," and in all subsequent editions she was spoken of as having only four

descents. Mr. Storer doubts even about the four, as no names of the dams are

given. Moreover, Clarissa bore "only two bulls and one heifer, and in the next

generation her progeny became extinct." Analogous cases of close interbreeding

are given in a pamphlet published by Mr. C. Macknight and Dr. H. Madden 'On

the True Principles of Breeding' Melbourne Australia 1865.) 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

(17/6. Mr. Willoughby Wood in 'Gardener's Chronicle' 1855 page 411; and 1860

page 270. See the very clear tables and pedigrees given in Nathusius

'Rindvieh' s. 72-77.), 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 much 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 (17/7. Mr. Wright

'Journal of Royal Agricult. Soc.' volume 7 1846 page 204. Mr. J. Downing (a

successful breeder of Shorthorns in Ireland) informs me that the raisers of

the great families of Shorthorns carefully conceal their sterility and want of

constitution. He adds that Mr. Bates, after he had bred his herd in-and-in for

some years, "lost in one season twenty-eight calves solely from want of

constitution.") 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 parts of England to be slaughtered (17/8.

'Youatt on Cattle' page 202.); 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. (17/9. 'Report

British Assoc. Zoolog. Sect.' 1838.) 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."

(17/10. Azara 'Quadrupedes du Paraguay' tome 2 pages 354, 368.) The decrease

in size from ancient times in the Chillingham and Hamilton cattle must have

been prodigious, for Professor Rutimeyer 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 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. (17/11. For the case of the Messrs. Brown see 'Gardener's

Chronicle' 1855 page 26. For the Foscote flock 'Gardener's Chronicle' 1860

page 416. For the Naz flock 'Bull. de la Soc. d'Acclimat.' 1860 page 477.)

Nevertheless, most great breeders of sheep have protested against close

interbreeding prolonged for too great a length of time. (17/12. Nathusius

'Rindvieh' s. 65; 'Youatt on Sheep' page 495.) 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" (17/13. 'Gardener's

Chronicle' 1861 page 631.); and what is probably of greater importance, the

separate flocks will have been exposed to somewhat different conditions.

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 of some kind is derived from this

practice. We have excellent evidence on this head from Mr. S. Druce (17/14.

'Journal R. Agricult. Soc.' volume 14 1853 page 212.), who gives in detail the

comparative numbers of four pure breeds and of a cross-breed which can be

supported on the same ground, and he gives their produce in fleece and

carcase. A high authority, Mr. Pusey, sums up the result in money value during

an equal length of time, namely (neglecting shillings), for Cotswolds 248

pounds, for Leicesters 223 pounds, for Southdowns 204 pounds, for Hampshire

Downs 264 pounds, and for the crossbred 293 pounds. A former celebrated

breeder, Lord Somerville, states that his half-breeds from Ryelands and

Spanish sheep were larger animals than either the pure Ryelands or pure

Spanish sheep. Mr. Spooner concludes his excellent Essay on Crossing by

asserting that there is a pecuniary advantage in judicious cross-breeding,

especially when the male is larger than the female. (17/15. Lord Somerville

'Facts on Sheep and Husbandry' page 6. Mr. Spooner in 'Journal of Royal

Agricult. Soc. of England' volume 20 part 2. See also an excellent paper on

the same subject in 'Gardener's Chronicle' 1860 page 321 by Mr. Charles

Howard.)

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 (17/16.

'Some Account of English Deer Parks' by Evelyn P. Shirley 1867.), 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 son, and

sometimes even from brothers and sisters. With greyhounds also there has been

much close interbreeding, but the best breeders agree that it may be carried

too far. (17/17. Stonehenge 'The Dog' 1867 pages 175-188.) But Sir J. Sebright

declares (17/18. 'The Art of Improving the Breed' etc. page 13. With respect

to Scotch deerhounds see Scrope 'Art of Deer Stalking' pages 350-353.), that

by breeding in-and-in, by which he means matching brothers and sisters, he has

actually seen the offspring of strong spaniels degenerate into 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 large price of all 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 formerly

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. A

great judge of rabbits (17/19. 'Cottage Gardener' 1861 page 327.) 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." They often

desert their young, so that it is necessary to have nurse-rabbits, but I do

not pretend to attribute all these evil results to close interbreeding.

(17/20. Mr. Huth gives ('The Marriage of Near Kin' 1875 page 302) from the

'Bulletin de l'Acad. R. de Med. de Belgique' (volume 9 1866 pages 287, 305),

several statements made by a M. Legrain with respect to crossing brother and

sister rabbits for five or six successive generations with no consequent evil

results. I was so much surprised at this account, and at M. Legrain's

invariable success in his experiments, that I wrote to a distinguished

naturalist in Belgium to inquire whether M. Legrain was a trustworthy

observer. In answer, I have heard that, as doubts were expressed about the

authenticity of these experiments, a commission of inquiry was appointed, and

that at a succeeding meeting of the Society ('Bull. de l'Acad. R. de Med. de

Belgique' 1867 3rd series tome 1 no. 1 to 5), Dr. Crocq reported "qu'il etait

materiellement impossible que M. Legrain ait fait les experiences qu'il

annonce." To this public accusation no satisfactory answer was made.)

With respect to 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." (17/21. Sidney's

edition of 'Youatt on the Pig' 1860 page 30; page 33 quotation from Mr. Druce;

page 29 on Lord Western's case.) 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 (17/22. 'Journal of Royal Agricult. Soc. of England' 1846 volume 7

page 205.) 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, 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 (17/23. 'Ueber Rindvieh' etc. s. 78. Col. Le Couteur, who has

done so much for the agriculture of Jersey, writes to me that from possessing

a fine breed of pigs he bred them very closely, twice pairing brothers and

sisters, but nearly all the young had fits and died suddenly.) 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; this 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 (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." (17/24. Sidney on the

'Pig' page 36. See also note page 34. Also Richardson on the 'Pig' 1847 page

26.)]

Almost all the animals as yet mentioned are gregarious, and the males must

frequently pair with their own daughters, for they expel the young males as

well as all intruders, until forced by old age and loss of strength to yield

to some stronger male. It is therefore not improbable that gregarious animals

may have been rendered less susceptible than non-social species to the evil

consequences of close interbreeding, so that they may be enabled to live in

herds without injury to their offspring. Unfortunately we do not know whether

an animal like the cat, which is not gregarious, would suffer from close

interbreeding in a greater degree than our other domesticated animals. But the

pig is not, as far as I can discover, strictly gregarious, and we have seen

that it appears eminently liable to the evil effects of close interbreeding.

Mr. Huth, in the case of the pig, attributes (page 285) these effects to their

having been "cultivated most for their fat," or to the selected individuals

having had a weak constitution; but we must remember that it is great breeders

who have brought forward the above cases, and who are far more familiar than

ordinary men can be, with the causes which are likely to interfere with the

fertility of their animals.

The effects of close interbreeding in the case of man is a difficult subject,

on which I will say but little. It has been discussed by various authors under

many points of view. (17/25. Dr. Dally has published an excellent article

(translated in the 'Anthropolog. Review' May 1864 page 65), criticising all

writers who have maintained that evil follows from consanguineous marriages.

No doubt on this side of the question many advocates have injured their cause

by inaccuracies: thus it has been stated (Devay 'Du Danger des Mariages' etc.

1862 page 141) that the marriages of cousins have been prohibited by the

legislature of Ohio; but I have been assured, in answer to inquiries made in

the United States, that this statement is a mere fable.) Mr. Tylor (17/26. See

his interesting work on the 'Early History of Man' 1865 chapter 10.) has shown

that with widely different races in the most distant quarters of the world,

marriages between relations--even between distant relations--have been

strictly prohibited. There are, however, many exceptions to the rule, which

are fully given by Mr. Huth (17/27. 'The Marriage of Near Kin' 1875. The

evidence given by Mr. Huth would, I think, have been even more valuable than

it is on this and some other points, if he had referred solely to the works of

men who had long resided in each country referred to, and who showed that they

possessed judgment and caution. See also Mr. W. Adam 'On Consanguinity in

Marriage' in the 'Fortnightly Review' 1865 page 710. Also Hofacker 'Ueber die

Eigenschaften' etc. 1828.) It is a curious problem how these prohibitions

arose during early and barbarous times. Mr. Tylor is inclined to attribute

them to the evil effects of consanguineous marriages having been observed; and

he ingeniously attempts to explain some apparent anomalies in the prohibition

not extending equally to the relations on 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 these views, seeing that incest is held

in abhorrence by savages such as those of Australia and South America (17/28.

Sir G. Grey 'Journal of Expeditions into Australia' volume 2 page 243; and

Dobrizhoffer 'On the Abipones of South America.'), who have no property to

bequeath, or fine moral feelings to confuse, and who are not likely to reflect

on distant evils to their progeny. According to Mr. Huth the feeling is the

indirect result of exogamy, inasmuch as when this practice ceased in any tribe

and it became endogamous, so that marriages were strictly confined to the same

tribe, it is not unlikely that a vestige of the former practice would still be

retained, so that closely-related marriages would be prohibited. With respect

to exogamy itself Mr. MacLennan believes that it arose from a scarcity of

women, owing to female infanticide, aided perhaps by other causes.

It has been clearly shown by Mr. Huth that there is no instinctive feeling in

man against incest any more than in gregarious animals. We know also how

readily any prejudice or feeling may rise to abhorrence, as shown by Hindus in

regard to objects causing defilement. Although there seems to be no strong

inherited feeling in mankind against incest, it seems possible that men during

primeval times may have been more excited by strange females than by those

with whom they habitually lived; in the same manner as according to Mr.

Cupples (17/29. 'Descent of Man' 2nd. edit page 524.), male deerhounds are

inclined towards strange females, while the females prefer dogs with whom they

have associated. If any such feeling formerly existed in man, this would have

led to a preference for marriages beyond the nearest kin, and might have been

strengthened by the offspring of such marriages surviving in greater numbers,

as analogy would lead us to believe would have occurred.

Whether consanguineous marriages, such as are permitted in civilised nations,

and which would not be considered as close interbreeding in the case of our

domesticated animals, cause any injury will never be known with certainty

until a census is taken with this object in view. My son, George Darwin, has

done what is possible at present by a statistical investigation (17/30.

'Journal of Statistical Soc.' June 1875 page 153; and 'Fortnightly Review'

June 1875.), and he has come to the conclusion, from his own researches and

those of Dr. Mitchell, that the evidence as to any evil thus caused is

conflicting, but on the whole points to the evil being very small.

[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. (17/31. 'The Art of Improving the Breed' page

13.) 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 animals; 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." (17/32. 'The Poultry Book' by W.B. Tegetmeier 1866 page 245.)

Mr. Wright states (17/33. 'Journal Royal Agricult. Soc.' 1846 volume 7 page

205; see also Ferguson on the Fowl pages 83, 317; see also 'The Poultry Book'

by Tegetmeier 1866 page 135 with respect to the extent to which cock-fighters

found that they could venture to breed in-and-in, viz., occasionally a hen

with her own son; "but they were cautious not to repeat the in-and-in

breeding.") 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 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. (17/34. 'The Poultry Book' by W.B. Tegetmeier 1866 page 79.)

An experienced writer (17/35. 'The Poultry Chronicle' 1854 volume 1 page 43.)

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." (17/36. 'The Poultry Book' by W.B. Tegetmeier 1866 page 79.)

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. Moreover, it is a generally received opinion that cross-

bred chickens are the hardiest and most easily reared. (17/37. 'The Poultry

Chronicle' volume 1 page 89.) Mr. Tegetmeier, who has carefully attended to

poultry of all breeds, says (17/38. 'The Poultry Book' 1866 page 210.) that

Dorking hens, allowed to run with Houdan or Creve-coeur 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 female common pheasant "are of far

larger size than either parent-bird." (17/39. Ibid 1866 page 167; and 'Poultry

Chronicle' volume 3 1855 page 15.) 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 notice

that, when size is one of the desired characters, as with pouters (17/40. 'A

Treatise on Fancy Pigeons' by J.M. Eaton page 56.) 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 (17/41. 'The Pigeon

Book' page 46.) 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 15/16 of trumpeter's blood; but then the experiment failed, for

"breeding so close stopped reproduction." The experienced Neumeister (17/42.

'Das Ganze der Taubenzucht' 1837 s. 18.) also asserts that the offspring from

dovecotes and various other breeds are "generally very fertile and hardy

birds:" so again MM. Boitard and Corbie (17/43. 'Les Pigeons' 1824 page 35.),

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 (17/44. 'Proc. Entomolog. Soc.' August 6, 1860

page 126.) that this instinct does not apply to drones, which are permitted to

enter any hive; so that there is no a 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. (17/45. 'Journal of Horticulture' 1861

pages 39, 77, 158; and 1864 page 206.)

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 with animals,

diminishes the evil from close interbreeding,--will occasionally be introduced

into each district.

With respect to individuals belonging to the same sub-variety, Gartner, whose

accuracy and experience exceeded that of all other observers, states (17/46.

'Beitrage zur Kenntniss der Befruchtung' 1844 s. 366.) 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, Fuchsia.

Herbert also says (17/47. 'Amaryllidaceae' page 371.), "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

ascertained that crossed offspring are more vigorous and robust than their

parents. (17/48. 'De la Fecondation' 2nd edition 1862 page 79.)

General statements of this kind, however, can seldom be fully trusted: I

therefore began a long series of experiments, continued for about ten years,

which will I think conclusively show the good effects of crossing two distinct

plants of the same variety, and the evil effects of long-continued self-

fertilisation. A clear light will thus be thrown on such questions, as why

flowers are almost invariably constructed so as to permit, or favour, or

necessitate the union of two individuals. We shall clearly understand why

monoecious and dioecious,--why dichogamous, dimorphic and trimorphic plants

exist, and many other such cases. I intend soon to publish an account of these

experiments, and I can here give only a few cases in illustration. The plan

which I followed was to grow plants in the same pot, or in pots of the same

size, or close together in the open ground; carefully to 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 of these experiments, the crossed plants yielded much more seed than the

self-fertilised plants; and I have never seen the 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 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. The growth of the plants raised from

the crossed and self-fertilised seed, were carefully observed from their

germination to maturity, in species belonging to fifty-two genera; and the

difference in their growth, and in withstanding unfavourable conditions, was

in most cases 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 two of the first cases 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 during several

succeeding generations, 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, 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 were thickly sown on opposite sides of a pot. The seedlings were at

first equal in height; but when the young crossed plants were half an inch,

the self-fertilised plants were only a quarter of an inch high. But this

degree of inequality did not last, 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 capsules. As in the former case,

the experiment was repeated during several succeeding generations. Had I not

watched these plants of 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, or from a distinct plant

growing in the same 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 (17/49. 'Memoire sur les

Cucurbitacees' pages 36, 28, 30.) 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 (17/50. Loudon's 'Gardener's

Mag.' volume 8 1832 page 52.): "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 (17/51. 'Transact. Hort. Soc.' volume 1 page 25.) believed that

his seedlings from crossed varieties of the apple exhibited increased vigour

and luxuriance; and M. Chevreul (17/52. 'Annal. des Sc. Nat.' 3rd series, Bot.

tome 6 page 189.) alludes to the extreme vigour of some of the crossed fruit-

trees raised by Sageret.

By crossing reciprocally the tallest and shortest peas, Knight (17/53.

'Philosophical Transactions' 1799 page 200.) 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.

Wiegmann (17/54. 'Ueber die Bastarderzeugung' 1828 s. 32, 33. For Mr.

Chaundy's case see Loudon's 'Gardener's Mag.' volume 7 1831 page 696.) 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 (17/55. 'Gardener's

Chronicle' 1846 page 601.) 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 (17/56. 'Philosoph. Transact.' 1799

page 201.), 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 (17/57. Quoted in 'Bull. Bot. Soc.

France' volume 2 1855 page 327.) 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, Kolreuter, Gartner, Herbert, Sageret, Lecoq, and

Naudin, have been struck with the wonderful vigour, height, size, tenacity of

life, precocity, and hardiness of their hybrid productions. Gartner (17/58.

Gartner 'Bastarderzeugung' s. 259, 518, 526 et seq.) sums up his conviction on

this head in the strongest terms. Kolreuter (17/59. 'Fortsetzung' 1763 s. 29;

'Dritte Fortsetzung' s. 44, 96; 'Act. Acad. St. Petersburg' 1782 part 2 page

251; 'Nova Acta' 1793 pages 391, 394; 'Nova Acta' 1795 pages 316, 323.) gives

numerous precise measurements of the weight and height of his hybrids in his

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 Gartner and Herbert; but the

most striking exceptions are given by Max Wichura (17/60. 'Die

Bastardbefruchtung' etc. 1865 s. 31, 41, 42.) who found that hybrid willows

were generally tender in constitution, dwarf, and short-lived.

Kolreuter explains the vast increase in the size of the roots, stems, etc., 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 (17/61. Max Wichura fully accepts this view

('Bastardbefruchtung' s. 43), as does the Rev. M.J. Berkeley in 'Journal of

Hort. Soc.' January 1866 page 70.); but Gartner (17/62. 'Bastarderzeugung' s.

394, 526, 528.) 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 (17/63. Kolreuter 'Nova Acta' 1795 page 316.) have been

transmitted to their progeny. The result in all cases is probably in part due

to the saving of nutriment and vital force through the sexual organs acting

imperfectly or not at all, 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 (17/64. Gartner 'Bastarderzeugung' s.

430.) that hybrids invariably breed with either pure parent, and not rarely

with a distinct species, more readily than with one another. Herbert is

inclined to explain even this fact by the advantage derived from a cross; but

Gartner 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 presently see,

show that a cross by itself undoubtedly tends to increase or re-establish the

fertility of hybrids.

The same law, namely, that the crossed offspring both of varieties and species

are larger than the parent-forms, holds good in the most striking manner with

hybrid animals as well as with mongrels. Mr. Bartlett, who has had such large

experience says, "Among all hybrids of vertebrated animals there is a marked

increase of size." He then enumerates many cases with mammals, including

monkeys, and with various families of birds. (17/65. Quoted by Dr. Murie in

'Proc. Zoolog. Soc.' 1870 page 40.)]

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 the foregoing, as self-sterility is not

here the result of long-continued 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

(17/66. 'Botanische Zeitung' January 1864 s. 3.):--

[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

and 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.

(17/67. 'Monatsbericht Akad. Wissen.' Berlin 1866 s. 372.)]

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 is the fact, 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 (17/68. International Hort. Congress, London

1866.) with pollen from other plants of the same species; and fifty-eight

capsules were obtained, including on an average 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. (17/69. 'Proc. Bot. Soc. of Edinburgh' May 1863: these

observations are given in abstract, and others are added, in the 'Journal of

Proc. of Linn. Soc.' volume 8 Bot. 1864 page 162.) Oncidium sphacelatum has

effective pollen, for Mr. Scott fertilised two distinct species with it; the

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. Robertson 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 effective, 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. Riviere

(17/70. Prof. Lecoq 'De la Fecondation' 2nd edition 1862 page 76.) 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.

As these orchids had been grown under unnatural conditions in hot-houses, I

concluded that their self-sterility was due to this cause. But Fritz Muller

informs me that at Desterro, in Brazil, he fertilised above one hundred

flowers of the above-mentioned Oncidium flexuosum, which is there endemic,

with its own pollen, and with that taken from distinct plants: all the former

were sterile, whilst those fertilised by pollen from any OTHER PLANT of the

same species were fertile. During the first three days there was no difference

in the action of the two kinds of pollen: that placed on stigma of the same

plant separated in the usual manner into grains, and emitted tubes which

penetrated the column, and the stigmatic chamber shut itself; but only those

flowers which had been fertilised by pollen taken from a distinct plant

produced seed-capsules. On a subsequent occasion these experiments were

repeated on a large scale with the same result. Fritz Muller found that four

other endemic species of Oncidium were in like manner utterly sterile with

their own pollen, but fertile with that from any other plant: some of them

likewise produced seed-capsules when impregnated with pollen of widely

distinct genera, such as Cyrtopodium, and Rodriguezia. Oncidium crispum,

however, differs from the foregoing species in varying much in its self-

sterility; some plants producing fine pods with their own pollen, others

failing to do so in two or three instances, Fritz Muller observed that the

pods produced by pollen taken from a distinct flower on the same plant, were

larger than those produced by the flower's own pollen. In Epidendrum

cinnabarinum, an orchid belonging to another division of the family, fine pods

were produced by the plant's own pollen, but they contained by weight only

about half as much seed as the capsules which had been fertilised by pollen

from a distinct plant, and in one instance from a distinct species; moreover,

a very large proportion, and in some cases nearly all the seeds produced by

the plant's own pollen, were destitute of an embryo. Some self-fertilised

capsules of a Maxillaria were in a similar state.

Another observation made by Fritz Muller is highly remarkable, namely, that

with various orchids the plant's own pollen not only fails to impregnate the

flower, but acts on the stigma, and is acted on, in an injurious or poisonous

manner. This is shown by the surface of the stigma in contact with the pollen,

and by the pollen itself becoming in from three to five days dark brown, and

then decaying. The discoloration and decay are not caused by parasitic

cryptograms, which were observed by Fritz Muller in only a single instance.

These changes are well shown by placing on the same stigma, at the same time,

the plant's own pollen and that from a distinct plant of the same species, or

of another species, or even of another and widely remote genus. Thus, on the

stigma of Oncidium flexuosum, the plant's own pollen and that from a distinct

plant were placed side by side, and in five days' time the latter was

perfectly fresh, whilst the plant's own pollen was brown. On the other hand,

when the pollen of a distinct plant of the Oncidium flexuosum and of the

Epidendrum zebra (nov. spec.?) were placed together on the same stigma, they

behaved in exactly the same manner, the grains separating, emitting tubes, and

penetrating the stigma, so that the two pollen-masses, after an interval of

eleven days, could not be distinguished except by the difference of their

caudicles, which, of course, undergo no change. Fritz Muller has, moreover,

made a large number of crosses between orchids belonging to distinct species

and genera, and he finds that in all cases when the flowers are not fertilised

their footstalks first begin to wither; and the withering slowly spreads

upwards until the germens fall off, after an interval of one or two weeks, and

in one instance of between six and seven weeks; but even in this latter case,

and in most other cases, the pollen and stigma remained in appearance fresh.

Occasionally, however, the pollen becomes brownish, generally on the external

surface, and not in contact with the stigma, as is invariably the case when

the plant's own pollen is applied.

Fritz Muller observed the poisonous action of the plant's own pollen in the

above-mentioned Oncidium flexuosum, O. unicorne, pubes (?), and in two other

unnamed species. Also in two species of Rodriguezia, in two of Notylia, in one

of Burlingtonia, and of a fourth genus in the same group. In all these cases,

except the last, it was proved that the flowers were, as might have been

expected, fertile with pollen from a distinct plant of the same species.

Numerous flowers of one species of Notylia were fertilised with pollen from

the same raceme; in two days' time they all withered, the germens began to

shrink, the pollen-masses became dark brown, and not one pollen-grain emitted

a tube. So that in this orchid the injurious action of the plant's own pollen

is more rapid than with Oncidium flexuosum. Eight other flowers on the same

raceme were fertilised with pollen from a distinct plant of the same species:

two of these were dissected, and their stigmas were found to be penetrated by

numberless pollen-tubes; and the germens of the other six flowers became well

developed. On a subsequent occasion many other flowers were fertilised with

their own pollen, and all fell off dead in a few days; whilst some flowers on

the same raceme which had been left simply unfertilised adhered and long

remained fresh. We have seen that in cross-unions between extremely distinct

orchids the pollen long remains undecayed; but Notylia behaved in this respect

differently; for when its pollen was placed on the stigma of Oncidium

flexuosum, both the stigma and pollen quickly became dark brown, in the same

manner as if the plant's own pollen had been applied.

Fritz Muller suggests that, as in all these cases the plant's own pollen is

not only impotent (thus effectually preventing self-fertilisation), but

likewise prevents, as was ascertained in the case of the Notylia and Oncidium

flexuosum, the action of subsequently applied pollen from a distinct

individual, it would be an advantage to the plant to have its own pollen

rendered more and more deleterious; for the germens would thus quickly be

killed, and dropping off, there would be no further waste in nourishing a part

which ultimately could be of no avail.

The same naturalist found in Brazil three plants of a Bignonia growing near

together. He fertilised twenty-nine flowerets on one of them with their own

pollen, and they did not set a single capsule. Thirty flowers were then

fertilised with pollen from a distinct plant, one of the three, and they

yielded only two capsules. Lastly, five flowers were fertilised with pollen

from a fourth plant growing at a distance, and all five produced capsules.

Fritz Muller thinks that the three plants which grew near one another were

probably seedlings from the same parent, and that from being closely related,

they acted very feebly on one another. This view is extremely probable, for he

has since shown in a remarkable paper (17/71. 'Jenaische Zeitschrift fur

Naturwiss.' b. 7 page 22 1872 and page 441 1873. A large part of this paper

has been translated in the 'American Naturalist' 1874 page 223.), that in the

case of some Brazilian species of Abutilon, which are self-sterile, and

between which he raised some complex hybrids, that these, if near relatives,

were much less fertile inter se, than when not closely related.]

We now come to cases closely analogous with those just given, but different in

so far that only certain individuals of the species are self-sterile. This

self-impotence does not depend on the pollen or ovules being in an unfit state

for fertilisation, for both have been found effective in union with other

plants of the same or of a distinct species. The fact of plants having

acquired so peculiar a constitution, that they can be fertilised more readily

by the pollen of a distinct species than by their own, is exactly the reverse

of what occurs with all ordinary species. For in the latter the two sexual

elements of the same individual plant are of course capable of freely acting

on each other; but are so constituted that they are more or less impotent when

brought into union with the sexual elements of a distinct species, and produce

more or less sterile hybrids.

[Gartner experimented on two plants of Lobelia fulgens, brought from separate

places, and found (17/72. 'Bastarderzeugung' s. 64, 357.) that their pollen

was good, for he fertilised with it L. cardinalis and syphilitica; their

ovules were likewise good, for they were fertilised by the pollen of these

same two species; but these two plants of L. fulgens could not be fertilised

by their own pollen, as can generally be effected with perfect ease with this

species. Again, the pollen of a plant of Verbascum nigrum grown in a pot was

found by Gartner (17/73. Ibid s. 357.) capable of fertilising V. lychnitis and

V. austriacum; the ovules could be fertilised by the pollen of V. thapsus; but

the flowers could not be fertilised by their own pollen. Kolreuter, also

(17/74. 'Zweite Fortsetzung' s. 10; 'Dritte Forts.' s. 40. Mr. Scott likewise

fertilised fifty-four flowers of Verbascum phoeniceum, including two

varieties, with their own pollen, and not a single capsule was produced. Many

of the pollen-grains emitted their tubes, but only a few of them penetrated

the stigmas; some slight effect however was produced, as many of the ovaries

became somewhat developed: 'Journal Asiatic Soc. Bengal' 1867 page 150.),

gives the case of three garden plants of Verbascum phoeniceum, which bore

during two years many flowers; these he fertilised successfully with the

pollen of no less than four distinct species, but they produced not a seed

with their own apparently good pollen; subsequently these same plants, and

others raised from seed, assumed a strangely fluctuating condition, being

temporarily sterile on the male or female side, or on both sides, and

sometimes fertile on both sides; but two of the plants were perfectly fertile

throughout the summer.

With Reseda odorata I have found certain individuals quite sterile with their

own pollen, and so it is with the indigenous Reseda lutea. The self-sterile

plants of both species were perfectly fertile when crossed with pollen from

any other individual of the same species. These observations will hereafter be

published in another work, in which I shall also show that seeds sent to me by

Fritz Muller produced by plants of Eschscholtzia californica which were quite

self-sterile in Brazil, yielded in this country plants which were only

slightly self-sterile.

It appears (17/75. Duvernoy quoted by Gartner 'Bastarderzeugung' s. 334) that

certain flowers on certain plants of Lilium candidum can be fertilised more

freely by pollen from a distinct individual than by their own. So, again, with

the varieties of the potato. Tinzmann (17/76. 'Gardener's Chronicle' 1846 page

183.), who made many trials with this plant, says that pollen from another

variety sometimes "exerts a powerful influence, and I have found sorts of

potatoes which would not bear seed from impregnation with the pollen of their

own flowers would bear it when impregnated with other pollen." It does not,

however, appear to have been proved that the pollen which failed to act on the

flower's own stigma was in itself good.

In the genus Passiflora it has long been known that several species do not

produce fruit, unless fertilised by pollen taken from distinct species: thus,

Mr. Mowbray (17/77. 'Transact. Hort. Soc.' volume 7 1830 page 95.) found that

he could not get fruit from P. alata and racemosa except by reciprocally

fertilising them with each other's pollen; and similar facts have been

observed in Germany and France. (17/78. Prof. Lecoq 'De la Fecondation' 1845

page 70; Gartner 'Bastarderzeugung' s. 64.) I have received two accounts of P.

quadrangularis never producing fruit from its own pollen, but doing so freely

when fertilised in one case with the pollen of P. coerulea, and in another

case with that of P. edulis. But in three other cases this species fruited

freely when fertilised with its own pollen; and the writer in one case

attributed the favourable result to the temperature of the house having been

raised from 5 deg to 10 deg Fahr. above the former temperature, after the

flowers were fertilised. (17/79. 'Gardener's Chronicle' 1868 page 1341.) With

respect to P. laurifolia, a cultivator of much experience has recently

remarked (17/80. 'Gardener's Chronicle' 1866 page 1068.) that the flowers

"must be fertilised with the pollen of P. coerulea, or of some other common

kind, as their own pollen will not fertilise them." But the fullest details on

this subject have been given by Messrs. Scott and Robertson Munro (17/81.

'Journal of Proc. of Linn. Soc.' volume 8 1864 page 1168. Mr. Robertson Munro

in 'Trans. Bot. Soc.' of Edinburgh volume 9 page 399.): plants of Passiflora

racemosa, coerulea, and alata flowered profusely during many years in the

Botanic Gardens of Edinburgh, and, though repeatedly fertilised with their own

pollen, never produced any seed; yet this occurred at once with all three

species when they were crossed together in various ways. In the case of P.

coerulea three plants, two of which grew in the Botanic Gardens, were all

rendered fertile, merely by impregnating each with pollen of one of the

others. The same result was attained in the same manner with P. alata, but

with only one plant out of three. As so many self-sterile species of

Passiflora have been mentioned, it should be stated that the flowers of the

annual P. gracilis are nearly as fertile with their own pollen as with that

from a distinct plant; thus sixteen flowers spontaneously self-fertilised

produced fruit, each containing on an average 21.3 seed, whilst fruit from

fourteen crossed flowers contained 24.1 seed.

Returning to P. alata, I have received (1866) some interesting details from

Mr. Robertson Munro. Three plants, including one in England, have already been

mentioned which were inveterately self-sterile, and Mr. Munro informs me of

several others which, after repeated trials during many years, have been found

in the same predicament. At some other places, however, this species fruits

readily when fertilised with its own pollen. At Taymouth Castle there is a

plant which was formerly grafted by Mr. Donaldson on a distinct species, name

unknown, and ever since the operation it has produced fruit in abundance by

its own pollen; so that this small and unnatural change in the state of this

plant has restored its self-fertility! Some of the seedlings from the Taymouth

Castle plant were found to be not only sterile with their own pollen, but with

each other's pollen, and with the pollen of distinct species. Pollen from the

Taymouth plant failed to fertilise certain plants of the same species, but was

successful on one plant in the Edinburgh Botanic Gardens. Seedlings were

raised from this latter union, and some of their flowers were fertilised by

Mr. Munro with their own pollen; but they were found to be as self-impotent as

the mother-plant had always proved, except when fertilised by the grafted

Taymouth plant, and except, as we shall see, when fertilised by her own

seedlings. For Mr. Munro fertilised eighteen flowers on the self-impotent

mother-plant with pollen from these her own self-impotent seedlings, and

obtained, remarkable as the fact is, eighteen fine capsules full of excellent

seed! I have met with no case in regard to plants which shows so well as this

of P. alata, on what small and mysterious causes complete fertility or

complete sterility depends.]

The facts hitherto given relate to the much-lessened or completely destroyed

fertility of pure species when impregnated with their own pollen, in

comparison with their fertility when impregnated by distinct individuals or

distinct species; but closely analogous facts have been observed with hybrids.

[Herbert states (17/82. 'Amaryllidaceae' 1837 page 371; 'Journal of Hort.

Soc.' volume 2 1847 page 19.) that having in flower at the same time nine

hybrid Hippeastrums, of complicated origin, descended from several species, he

found that "almost every flower touched with pollen from another cross

produced seed abundantly, and those which were touched with their own pollen

either failed entirely, or formed slowly a pod of inferior size, with fewer

seeds." In the 'Horticultural Journal' he adds that "the admission of the

pollen of another cross-bred Hippeastrum (however complicated the cross) to

any one flower of the number, is almost sure to check the fructification of

the others." In a letter written to me in 1839, Dr. Herbert says that he had

already tried these experiments during five consecutive years, and he

subsequently repeated them, with the same invariable result. He was thus led

to make an analogous trial on a pure species, namely, on the Hippeastrum

aulicum, which he had lately imported from Brazil: this bulb produced four

flowers, three of which were fertilised by their own pollen, and the fourth by

the pollen of a triple cross between H. bulbulosum, reginae, and vittatum; the

result was, that "the ovaries of the three first flowers soon ceased to grow,

and after a few days perished entirely: whereas the pod impregnated by the

hybrid made vigorous and rapid progress to maturity, and bore good seed, which

vegetated freely." This is, indeed, as Herbert remarks, "a strange truth," but

not so strange as it then appeared.

As a confirmation of these statements, I may add that Mr. M. Mayes (17/83.

Loudon's 'Gardener's Magazine' volume 11 1835 page 260.) after much experience

in crossing the species of Amaryllis (Hippeastrum), says, "neither the species

nor the hybrids will, we are well aware, produce seed so abundantly from their

own pollen as from that of others." So, again, Mr. Bidwell, in New South Wales

(17/84. 'Gardener's Chronicle' 1850 page 470.) asserts that Amaryllis

belladonna bears many more seeds when fertilised by the pollen of Brunswigia

(Amaryllis of some authors) josephinae or of B. multiflora, than when

fertilised by its own pollen. Mr. Beaton dusted four flowers of a Cyrtanthus

with their own pollen, and four with the pollen of Vallota (Amaryllis)

purpurea; on the seventh day "those which received their own pollen slackened

their growth, and ultimately perished; those which were crossed with the

Vallota held on." (17/85. 'Journal Hort. Soc.' volume 5 page 135. The

seedlings thus raised were given to the Hort. Soc.; but I find, on inquiry,

that they unfortunately died the following winter.) These latter cases,

however, relate to uncrossed species, like those before given with respect to

Passiflora, Orchids, etc., and are here referred to only because the plants

belong to the same group of Amaryllidaceae.

In the experiments on the hybrid Hippeastrums, if Herbert had found that the

pollen of two or three kinds alone had been more efficient on certain kinds

than their own pollen, it might have been argued that these, from their mixed

parentage, had a closer mutual affinity than the others; but this explanation

is inadmissible, for the trials were made reciprocally backwards and forwards

on nine different hybrids; and a cross, whichever way taken, always proved

highly beneficial. I can add a striking and analogous case from experiments

made by the Rev. A. Rawson, of Bromley Common, with some complex hybrids of

Gladiolus. This skilful horticulturist possessed a number of French varieties,

differing from each other only in the colour and size of the flowers, all

descended from Gandavensis, a well-known old hybrid, said to be descended from

G. natalensis by the pollen of G. oppositiflorus. (17/86. Mr. D. Beaton in

'Journal of Hort.' 1861 page 453. Lecoq however ('De la Fecond.' 1862 page

369) states that this hybrid is descended from G. psittacinus and cardinalis;

but this is opposed to Herbert's experience, who found that the former species

could not be crossed.) Mr. Rawson, after repeated trials, found that none of

the varieties would set seed with their own pollen, although taken from

distinct plants of the same variety (which had, of course, been propagated by

bulbs), but that they all seeded freely with pollen from any other variety. To

give two examples: Ophir did not produce a capsule with its own pollen, but

when fertilised with that of Janire, Brenchleyensis, Vulcain and Linne, it

produced ten fine capsules; but the pollen of Ophir was good, for when Linne

was fertilised by it seven capsules were produced. This latter variety, on the

other hand, was utterly barren with its own pollen, which we have seen was

perfectly efficient on Ophir. Altogether, Mr. Rawson, in the year 1861

fertilised twenty-six flowers borne by four varieties with pollen taken from

other varieties, and every single flower produced a fine seed-capsule; whereas

fifty-two flowers on the same plants, fertilised at the same time with their

own pollen, did not yield a single seed-capsule. Mr. Rawson fertilised, in

some cases, the alternate flowers, and in other cases all those down one side

of the spike, with pollen of other varieties, and the remaining flowers with

their own pollen. I saw these plants when the capsules were nearly mature, and

their curious arrangement at once brought full conviction to the mind that an

immense advantage had been derived from crossing these hybrids.

Lastly, I have heard from Dr. E. Bornet, of Antibes, who has made numerous

experiments in crossing the species of Cistus, but has not yet published the

results, that, when any of these hybrids are fertile, they may be said to be,

in regard to function, dioecious; "for the flowers are always sterile when the

pistil is fertilised by pollen taken from the same flower or from flowers on

the same plant. But they are often fertile if pollen be employed from a

distinct individual of the same hybrid nature, or from a hybrid made by a

reciprocal cross."]

CONCLUSION.

That plants should be self-sterile, although both sexual elements are in a fit

state for reproduction, appears at first sight opposed to all analogy. With

respect to the species, all the individuals of which are in this state,

although living under their natural conditions, we may conclude that their

self-sterility has been acquired for the sake of effectually preventing self-

fertilisation. The case is closely analogous with that of dimorphic and

trimorphic or heterostyled plants, which can be fully fertilised only by

plants belonging to a different form, and not, as in the foregoing cases,

indifferently by any other individual of the species. Some of these hetero-

styled plants are completely sterile with pollen taken from the same plant or

from the same form. With respect to species living under their natural

conditions, of which only certain individuals are self-sterile (as with Reseda

lutea), it is probable that these have been rendered self-sterile to ensure

occasional cross-fertilisation, whilst other individuals have remained self-

fertile to ensure the propagation of the species. The case seems to be

parallel with that of plants which produce, as Hermann Muller has discovered,

two forms--one bearing more conspicuous flowers with their structure adapted

for cross-fertilisation by insects, and the other form with less conspicuous

flowers adapted for self-fertilisation. The self-sterility, however, of some

of the foregoing plants is incidental on the conditions to which they have

been subjected, as with the Eschscholtzia, the Verbascum phoeniceum (the

sterility of which varied according to the season), and with the Passiflora

alata, which recovered its self-fertility when grafted on a different stock.

It is interesting to observe in the above several cases the graduated series

from plants which, when fertilised by their own pollen, yield the full number

of seeds, but with the seedlings a little dwarfed in stature--to plants which

when self-fertilised yield few seeds--to those which yield none, but have

their ovaria somewhat developed--and, lastly, to those in which the plant's

own pollen and stigma mutually act on one another like poison. It is also

interesting to observe on how slight a difference in the nature of the pollen

or of the ovules complete self-sterility or complete self-fertility must

depend in some of the above cases. Every individual of the self-sterile

species appears to be capable of producing the full complement of seed when

fertilised by the pollen of any other individual (though judging from the

facts given with respect to Abutilon the nearest kin must be excepted); but

not one individual can be fertilised by its own pollen. As every organism

differs in some slight degree from every other individual of the same species,

so no doubt it is with their pollen and ovules; and in the above cases we must

believe that complete self-sterility and complete self-fertility depend on

such slight differences in the ovules and pollen, and not their having been

differentiated in some special manner in relation to one another; for it is

impossible that the sexual elements of many thousand individuals should have

been specialised in relation to every other individual. In some, however, of

the above cases, as with certain Passifloras, an amount of differentiation

between the pollen and ovules sufficient for fertilisation is gained only by

employing pollen from a distinct species; but this is probably the result of

such plants having been rendered somewhat sterile from the unnatural

conditions to which they have been exposed.

Exotic animals confined in menageries are sometimes in nearly the same state

as the above-described self-impotent plants; for, as we shall see in the

following chapter, certain monkeys, the larger carnivora, several finches,

geese, and pheasants, cross together, quite as freely as, or even more freely

than the individuals of the same species breed together. Cases will, also, be

given of sexual incompatibility between certain, male and female domesticated

animals, which, nevertheless, are fertile when matched with any other

individual of the same kind.

In the early part of this chapter it was shown that the crossing of

individuals belonging to distinct families of the same race, or to different

races or species, gives increased size and constitutional vigour to the

offspring, and, except in the case of crossed species, increased fertility.

The evidence rests on the universal testimony of breeders (for it should be

observed that I am not here speaking of the evil results of close

interbreeding), and is practically exemplified in the higher value of cross-

bred animals for immediate consumption. The good results of crossing have also

been demonstrated with some animals and with numerous plants, by actual weight

and measurement. Although animals of pure blood will obviously be deteriorated

by crossing, as far as their characteristic qualities are concerned, there

seems to be no exception to the rule that advantages of the kind just

mentioned are thus gained, even when there has not been any previous close

interbreeding; and the rule applies to such animals as cattle and sheep, which

can long resist breeding in-and-in between the nearest blood-relations.

In the case of crossed species, although size, vigour, precocity, and

hardiness are, with rare exceptions, gained, fertility, in a greater or less

degree, is lost; but the gain in the above respects can hardly be attributed

to the principle of compensation; for there is no close parallelism between

the increased size and vigour of hybrid offspring and their sterility.

Moreover, it has been clearly proved that mongrels which are perfectly fertile

gain these same advantages as well as sterile hybrids.

With the higher animals no special adaptations for ensuring occasional crosses

between distinct families seem to exist. The eagerness of the males, leading

to severe competition between them, is sufficient; for even with gregarious

animals, the old and dominant males will be dispossessed after a time and it

would be a mere chance if a closely related member of the same family were to

be the victorious successor. The structure of many of the lower animals, when

they are hermaphrodites, is such as to prevent the ovules being fertilised by

the male element of the same individual; so that the concourse of two

individuals is necessary. In other cases the access of the male element of a

distinct individual is at least possible. With plants, which are affixed to

the ground and cannot wander from place to place like animals, the numerous

adaptations for cross-fertilisation are wonderfully perfect, as has been

admitted by every one who has studied the subject.

The evil consequences of long-continued close interbreeding are not so easily

recognised as the good effects from crossing, for the deterioration is

gradual. Nevertheless, it is the general opinion of those who have had most

experience, especially with animals which propagate quickly, that evil does

inevitably follow sooner or later, but at different rates with different

animals. No doubt a false belief may, like a superstition, prevail widely; yet

it is difficult to suppose that so many acute observers have all been deceived

at the expense of much cost and trouble. A male animal may sometimes be paired

with his daughter, granddaughter, and so on, even for seven generations,

without any manifest bad result: but the experiment has never been tried of

matching brothers and sisters, which is considered the closest form of

interbreeding, for an equal number of generations. There is good reason to

believe that by keeping the members of the same family in distinct bodies,

especially if exposed to somewhat different conditions of life, and by

occasionally crossing these families, the evil results of interbreeding may be

much diminished or quite eliminated. These results are loss of constitutional

vigour, size, and fertility; but there is no necessary deterioration in the

general form of the body, or in other good qualities. We have seen that with

pigs first-rate animals have been produced after long-continued close

interbreeding, though they had become extremely infertile when paired with

their near relations. The loss of fertility, when it occurs, seems never to be

absolute, but only relative to animals of the same blood; so that this

sterility is to a certain extent analogous with that of self-impotent plants

which cannot be fertilised by their own pollen, but are perfectly fertile with

pollen of any other individual of the same species. The fact of infertility of

this peculiar nature being one of the results of long-continued interbreeding,

shows that interbreeding does not act merely by combining and augmenting

various morbid tendencies common to both parents; for animals with such

tendencies, if not at the time actually ill, can generally propagate their

kind. Although offspring descended from the nearest blood-relations are not

necessarily deteriorated in structure, yet some authors believe that they are

eminently liable to malformations; and this is not improbable, as everything

which lessens the vital powers acts in this manner. Instances of this kind

have been recorded in the case of pigs, bloodhounds, and some other animals.

Finally, when we consider the various facts now given which plainly show that

good follows from crossing, and less plainly that evil follows from close

interbreeding, and when we bear in mind that with very many organisms

elaborate provisions have been made for the occasional union of distinct

individuals, the existence of a great law of nature is almost proved; namely,

that the crossing of animals and plants which are not closely related to each

other is highly beneficial or even necessary, and that interbreeding prolonged

during many generations is injurious.