Although this would be rash, it would, I think, be still rasher, seeing that sterility is no more incidental to _all_ cross-bred productions than it is to all organic beings when captured by man, to a.s.sert that the sterility of certain hybrids proved a distinct creation of their parents.
{248} Animals seem more often made sterile by being taken out of their native condition than plants, and so are more sterile when crossed.We have one broad fact that sterility in hybrids is not closely related to external difference, and these are what man alone gets by selection.
{249} See _Var. under Dom._, Ed. ii. vol. II. p. 132; for the case of the cheetah see _loc cit._ p. 133.
{250} _Var. under Dom._, Ed. ii. vol. II. p. 148.
{251} Quoted in the _Origin_, Ed. i. p. 9.
{252} See _Var. under Dom._, Ed. ii. vol. II. p. 147.
{253} _Var. under Dom._, Ed. ii. vol. II. p. 89.
{254} See _Var. under Dom._, Ed. ii. vol. II. p. 147.
{255} _Origin_, Ed. i. p. 267, vi. p. 392. This is the principle experimentally investigated in the author"s _Cross-and Self-Fertilisation_.
But it may be objected{256} (however little the sterility of certain hybrids is connected with the distinct creations of species), how comes it, if species are only races produced by natural selection, that when crossed they so frequently produce sterile offspring, whereas in the offspring of those races confessedly produced by the arts of man there is no one instance of sterility. There is not much difficulty in this, for the races produced by the natural means above explained will be slowly but steadily selected; will be adapted to various and diverse conditions, and to these conditions they will be rigidly confined for immense periods of time; hence we may suppose that they would acquire different const.i.tutional peculiarities adapted to the stations they occupy; and on the const.i.tutional differences between species their sterility, according to the best authorities, depends. On the other hand man selects by external appearance{257}; from his ignorance, and from not having any test at least comparable in delicacy to the natural struggle for food, continued at intervals through the life of each individual, he cannot eliminate fine shades of const.i.tution, dependent on invisible differences in the fluids or solids of the body; again, from the value which he attaches to each individual, he a.s.serts his utmost power in contravening the natural tendency of the most vigorous to survive. Man, moreover, especially in the earlier ages, cannot have kept his conditions of life constant, and in later ages his stock pure.
Until man selects two varieties from the same stock, adapted to two climates or to other different external conditions, and confines each rigidly for one or several thousand years to such conditions, always selecting the individuals best adapted to them, he cannot be said to have even commenced the experiment. Moreover, the organic beings which man has longest had under domestication have been those which were of the greatest use to him, and one chief element of their usefulness, especially in the earlier ages, must have been their capacity to undergo sudden transportals into various climates, and at the same time to retain their fertility, which in itself implies that in such respects their const.i.tutional peculiarities were not closely limited. If the opinion already mentioned be correct, that most of the domestic animals in their present state have descended from the fertile commixture of wild races or species, we have indeed little reason now to expect infertility between any cross of stock thus descended.
{256} _Origin_, Ed. i. p. 268, vi. p. 398.
{257} Mere difference of structure no guide to what will or will not cross. First step gained by races keeping apart. It is worthy of remark, that as many organic beings, when taken by man out of their natural conditions, have their reproductive system affected as to be incapable of propagation, so, we saw in the first chapter, that although organic beings when taken by man do propagate freely, their offspring after some generations vary or sport to a degree which can only be explained by their reproductive system being some way affected. Again, when species cross, their offspring are generally sterile; but it was found by Kolreuter that when hybrids are capable of breeding with either parent, or with other species, that their offspring are subject after some generations to excessive variation{258}. Agriculturists, also, affirm that the offspring from mongrels, after the first generation, vary much. Hence we see that both sterility and variation in the succeeding generations are consequent both on the removal of individual species from their natural states and on species crossing. The connection between these facts may be accidental, but they certainly appear to elucidate and support each other,--on the principle of the reproductive system of all organic beings being eminently sensitive to any disturbance, whether from removal or commixture, in their const.i.tutional relations to the conditions to which they are exposed.{258} _Origin_, Ed. i. p. 272, vi. p. 404.
_Points of Resemblance between "Races" and "Species{259}."_
{259} This section seems not to correspond closely with any in the _Origin_, Ed. i.; in some points it resembles pp. 15, 16, also the section on a.n.a.logous variation in distinct species, _Origin_, Ed.
i. p. 159, vi. p. 194.
Races and reputed species agree in some respects, although differing from causes which, we have seen, we can in some degree understand, in the fertility and "trueness" of their offspring. In the first place, there is no clear sign by which to distinguish races from species, as is evident from the great difficulty experienced by naturalists in attempting to discriminate them. As far as external characters are concerned, many of the races which are descended from the same stock differ far more than true species of the same genus; look at the willow-wrens, some of which skilful ornithologists can hardly distinguish from each other except by their nests; look at the wild swans, and compare the distinct species of these genera with the races of domestic ducks, poultry, and pigeons; and so again with plants, compare the cabbages, almonds, peaches and nectarines, &c. with the species of many genera. St Hilaire has even remarked that there is a greater difference in size between races, as in dogs (for he believes all have descended from one stock), than between the species of any one genus; nor is this surprising, considering that amount of food and consequently of growth is the element of change over which man has most power. I may refer to a former statement, that breeders believe the growth of one part or strong action of one function causes a decrease in other parts; for this seems in some degree a.n.a.logous to the law of "organic compensation{260}," which many naturalists believe holds good.
To give an instance of this law of compensation,--those species of Carnivora which have the canine teeth greatly developed have certain molar teeth deficient; or again, in that division of the Crustaceans in which the tail is much developed, the thorax is little so, and the converse. The points of difference between different races is often strikingly a.n.a.logous to that between species of the same genus: trifling spots or marks of colour{261} (as the bars on pigeons" wings) are often preserved in races of plants and animals, precisely in the same manner as similar trifling characters often pervade all the species of a genus, and even of a family. Flowers in varying their colours often become veined and spotted and the leaves become divided like true species: it is known that the varieties of the same plant never have red, blue and yellow flowers, though the hyacinth makes a very near approach to an exception{262}; and different species of the same genus seldom, though sometimes they have flowers of these three colours. Dun-coloured horses having a dark stripe down their backs, and certain domestic a.s.ses having transverse bars on their legs, afford striking examples of a variation a.n.a.logous in character to the distinctive marks of other species of the same genus.
{260} The law of compensation is discussed in the _Origin_, Ed. i.
p. 147, vi. p. 182.
{261} Boitard and Corbie on outer edging red in tail of bird,--so bars on wing, white or black or brown, or white edged with black or : a.n.a.logous to marks running through genera but with different colours. Tail coloured in pigeons.{262} Oxalis and Gentian. _External characters of Hybrids and Mongrels._
There is, however, as it appears to me, a more important method of comparison between species and races, namely the character of the offspring{263} when species are crossed and when races are crossed: I believe, in no one respect, except in sterility, is there any difference. It would, I think, be a marvellous fact, if species have been formed by distinct acts of creation, that they should act upon each other in uniting, like races descended from a common stock. In the first place, by repeated crossing one species can absorb and wholly obliterate the characters of another, or of several other species, in the same manner as one race will absorb by crossing another race. Marvellous, that one act of creation should absorb another or even several acts of creation! The offspring of species, that is hybrids, and the offspring of races, that is mongrels, resemble each other in being either intermediate in character (as is most frequent in hybrids) or in resembling sometimes closely one and sometimes the other parent; in both the offspring produced by the same act of conception sometimes differ in their degree of resemblance; both hybrids and mongrels sometimes retain a certain part or organ very like that of either parent, both, as we have seen, become in succeeding generations variable; and this tendency to vary can be transmitted by both; in both for many generations there is a strong tendency to reversion to their ancestral form. In the case of a hybrid laburnum and of a supposed mongrel vine different parts of the same plants took after each of their two parents. In the hybrids from some species, and in the mongrel of some races, the offspring differ according as which of the two species, or of the two races, is the father (as in the common mule and hinny) and which the mother. Some races will breed together, which differ so greatly in size, that the dam often perishes in labour; so it is with some species when crossed; when the dam of one species has borne offspring to the male of another species, her succeeding offspring are sometimes stained (as in Lord Morton"s mare by the quagga, wonderful as the fact{264} is) by this first cross; so agriculturists positively affirm is the case when a pig or sheep of one breed has produced offspring by the sire of another breed.
{263} This section corresponds roughly to that on _Hybrids and Mongrels compared independently of their fertility_, _Origin_, Ed.
i. p. 272, vi. p. 403. The discussion on Gartner"s views, given in the _Origin_, is here wanting. The brief mention of prepotency is common to them both.
{264} See _Animals and Plants_, Ed. ii. vol. I. p. 435. The phenomenon of _Telegony_, supposed to be established by this and similar cases, is now generally discredited in consequence of Ewart"s experiments.
_Summary of second chapter_{265}.
{265} The section on p. 109 is an appendix to the summary.
Let us sum up this second chapter. If slight variations do occur in organic beings in a state of nature; if changes of condition from geological causes do produce in the course of ages effects a.n.a.logous to those of domestication on any, however few, organisms; and how can we doubt it,--from what is actually known, and from what may be presumed, since thousands of organisms taken by man for sundry uses, and placed in new conditions, have varied. If such variations tend to be hereditary; and how can we doubt it,--when we see shades of expression, peculiar manners, monstrosities of the strangest kinds, diseases, and a mult.i.tude of other peculiarities, which characterise and form, being inherited, the endless races (there are 1200 kinds of cabbages{266}) of our domestic plants and animals. If we admit that every organism maintains its place by an almost periodically recurrent struggle; and how can we doubt it,--when we know that all beings tend to increase in a geometrical ratio (as is instantly seen when the conditions become for a time more favourable); whereas on an average the amount of food must remain constant, if so, there will be a natural means of selection, tending to preserve those individuals with any slight deviations of structure more favourable to the then existing conditions, and tending to destroy any with deviations of an opposite nature. If the above propositions be correct, and there be no law of nature limiting the possible amount of variation, new races of beings will,--perhaps only rarely, and only in some few districts,--be formed.
{266} I do not know the authority for this statement.
_Limits of Variation._
That a limit to variation does exist in nature is a.s.sumed by most authors, though I am unable to discover a single fact on which this belief is grounded{267}. One of the commonest statements is that plants do not become acclimatised; and I have even observed that kinds not raised by seed, but propagated by cuttings, &c., are instanced. A good instance has, however, been advanced in the case of kidney beans, which it is believed are now as tender as when first introduced. Even if we overlook the frequent introduction of seed from warmer countries, let me observe that as long as the seeds are gathered promiscuously from the bed, without continual observation and _careful_ selection of those plants which have stood the climate best during their whole growth, the experiment of acclimatisation has hardly been begun. Are not all those plants and animals, of which we have the greatest number of races, the oldest domesticated? Considering the quite recent progress{268} of systematic agriculture and horticulture, is it not opposed to every fact, that we have exhausted the capacity of variation in our cattle and in our corn,--even if we have done so in some trivial points, as their fatness or kind of wool? Will any one say, that if horticulture continues to flourish during the next few centuries, that we shall not have numerous new kinds of the potato and Dahlia? But take two varieties of each of these plants, and adapt them to certain fixed conditions and prevent any cross for 5000 years, and then again vary their conditions; try many climates and situations; and who{269} will predict the number and degrees of difference which might arise from these stocks? I repeat that we know nothing of any limit to the possible amount of variation, and therefore to the number and differences of the races, which might be produced by the natural means of selection, so infinitely more efficient than the agency of man. Races thus produced would probably be very "true"; and if from having been adapted to different conditions of existence, they possessed different const.i.tutions, if suddenly removed to some new station, they would perhaps be sterile and their offspring would perhaps be infertile. Such races would be undistinguishable from species. But is there any evidence that the species, which surround us on all sides, have been thus produced? This is a question which an examination of the economy of nature we might expect would answer either in the affirmative or negative{270}.
{267} In the _Origin_ no limit is placed to variation as far as I know.
{268} History of pigeons shows increase of peculiarities during last years.{269} Compare an obscure pa.s.sage in the Essay of 1842, p. 14.
{270} Certainly ought to be here introduced, viz., difficulty in forming such organ, as eye, by selection. CHAPTER III
ON THE VARIATION OF INSTINCTS AND OTHER MENTAL ATTRIBUTES UNDER DOMESTICATION AND IN STATE OF NATURE; ON THE DIFFICULTIES IN THIS SUBJECT; AND ON a.n.a.lOGOUS DIFFICULTIES WITH RESPECT TO CORPOREAL STRUCTURES
_Variation of mental attributes under domestication._
I have as yet only alluded to the mental qualities which differ greatly in different species. Let me here premise that, as will be seen in the Second Part, there is no evidence and consequently no attempt to show that _all_ existing organisms have descended from any one common parent-stock, but that only those have so descended which, in the language of naturalists, are clearly related to each other. Hence the facts and reasoning advanced in this chapter do not apply to the first origin of the senses{271}, or of the chief mental attributes, such as of memory, attention, reasoning, &c., &c., by which most or all of the great related groups are characterised, any more than they apply to the first origin of life, or growth, or the power of reproduction. The application of such facts as I have collected is merely to the differences of the primary mental qualities and of the instincts in the species{272} of the several great groups. In domestic animals every observer has remarked in how great a degree, in the individuals of the same species, the dispositions, namely courage, pertinacity, suspicion, restlessness, confidence, temper, pugnaciousness, affection, care of their young, sagacity, &c., &c., vary. It would require a most able metaphysician to explain how many primary qualities of the mind must be changed to cause these diversities of complex dispositions. From these dispositions being inherited, of which the testimony is unanimous, families and breeds arise, varying in these respects. I may instance the good and ill temper of different stocks of bees and of horses,--the pugnacity and courage of game fowls,--the pertinacity of certain dogs, as bull-dogs, and the sagacity of others,--for restlessness and suspicion compare a wild rabbit reared with the greatest care from its earliest age with the extreme tameness of the domestic breed of the same animal. The offspring of the domestic dogs which have run wild in Cuba{273}, though caught quite young, are most difficult to tame, probably nearly as much so as the original parent-stock from which the domestic dog descended. The habitual "_periods_" of different families of the same species differ, for instance, in the time of year of reproduction, and the period of life when the capacity is acquired, and the hour of roosting (in Malay fowls), &c., &c. These periodical habits are perhaps essentially corporeal, and may be compared to nearly similar habits in plants, which are known to vary extremely. Consensual movements (as called by Muller) vary and are inherited,--such as the cantering and ambling paces in horses, the tumbling of pigeons, and perhaps the handwriting, which is sometimes so similar between father and sons, may be ranked in this cla.s.s. _Manners_, and even tricks which perhaps are only _peculiar_ manners, according to W. Hunter and my father, are distinctly inherited in cases where children have lost their parent in early infancy. The inheritance of expression, which often reveals the finest shades of character, is familiar to everyone.
{271} A similar proviso occurs in the chapter on instinct in _Origin_, Ed. i. p. 207, vi. p. 319.
{272} The discussion occurs later in Chapter VII of the _Origin_, Ed. i. than in the present Essay, where moreover it is fuller in some respects.
{273} In the margin occurs the name of Poeppig. In _Var. under Dom._, Ed. ii. vol. I. p. 28, the reference to Poeppig on the Cuban dogs contains no mention of the wildness of their offspring.
Again the tastes and pleasures of different breeds vary, thus the shepherd-dog delights in chasing the sheep, but has no wish to kill them,--the terrier (see Knight) delights in killing vermin, and the spaniel in finding game. But it is impossible to separate their mental peculiarities in the way I have done: the tumbling of pigeons, which I have instanced as a consensual movement, might be called a trick and is a.s.sociated with a taste for flying in a close flock at a great height.
Certain breeds of fowls have a taste for roosting in trees. The different actions of pointers and setters might have been adduced in the same cla.s.s, as might the peculiar _manner_ of hunting of the spaniel.
Even in the same breed of dogs, namely in fox-hounds, it is the fixed opinion of those best able to judge that the different pups are born with different tendencies; some are best to find their fox in the cover; some are apt to run straggling, some are best to make casts and to recover the lost scent, &c.; and that these peculiarities undoubtedly are transmitted to their progeny. Or again the tendency to point might be adduced as a distinct habit which has become inherited,--as might the tendency of a true sheep dog (as I have been a.s.sured is the case) to run round the flock instead of directly at them, as is the case with other young dogs when attempted to be taught. The "transandantes" sheep{274} in Spain, which for some centuries have been yearly taken a journey of several hundred miles from one province to another, know when the time comes, and show the greatest restlessness (like migratory birds in confinement), and are prevented with difficulty from starting by themselves, which they sometimes do, and find their own way. There is a case on good evidence{275} of a sheep which, when she lambed, would return across a mountainous country to her own birth-place, although at other times of year not of a rambling disposition. Her lambs inherited this same disposition, and would go to produce their young on the farm whence their parent came; and so troublesome was this habit that the whole family was destroyed.