THE VARIATION OF ANIMALS AND PLANTS UNDER DOMESTICATION.*

[SECOND NOTICE.]

NEARLY a hundred pages are devoted to a minute examination of the evidence upon which Mr. Darwin's conclusion is based, that all our breeds of domestic pigeons are descended from the columba livia, or blue-rock pigeon. We think that every reader who is capable of weighing the value of evidence must come to the conclusion that here Mr. Darwin completely establishes his case ; and if it be so, those who can look at the broader question without trepidation must inevitably admit that if anything so different in details of structure as can be found in the blue-rock and fan-tail or pouter pigeons, the further extension of his con- clusions is but a matter of time and perseverance. The initial difficulty in the way of considering this in its broader aspects will be removed at once, if the reader will bear in mind, that time is the most essential element of change of every kind, and that the important changes seen in pigeons have been artificially and clumsily induced by man in a comparatively very short time ; whilst for the modifications of structure induced by natural selec- tion, following those induced by purely natural circumstances, there is an extension of time which we not only cannot measure, but positively can form no conception of.

This part of Mr. Darwin's book seems almost to form a com- plete handbook for pigeon-fanciers, and some of the facts he has detailed are not only important, but curious. Thus he tells us that the especial characters for which each breed is valued are eminently variable, as in the Fantail, where the number and direction of the tail-feathers, the carriage of the body, and the degree of trembling are all highly variable. It is, first of all, almost unin- telligible why fanciers, who, of course, are wholly unscientific in their object of selection, should select trembling as a point. We are ignorant of the ways of pigeons, but have little doubt that this trembling is a sexual peculiarity ; and as it is variable, there can be no limit to the power of its extension by:artificial selection, so that a new pigeon-breed of "Shakers " might be produced. It

• Animals and Plants under Domegicatim By Charles Darwin, F.E.S. London: Murray.

will, no doubt, be regarded as a very humiliating fact, by a certain class of modern philosophers of both sexes, that in pigeons a "high degree of merit is rarer in the female than in the male ;" and that "if a cock and hen Tumbler were of equal merit, the hen would be worth double the value of the cock."

This is all the more remarkable, and will probably be found still more applicable to man than to pigeons, as "it is found that in domesticated pigeons certain sexual differences are found to be developed and to increase with age," whilst "there is no sensible difference at any age between the two sexes in the aboriginal rock-pigeon."

Mr. Darwin carries his minute observations over fowls, ducks, geese, peacocks, canaries, gold-fish, hive-bees, and silk-moths, and makes all contribute their share of information. Of canaries he records one most curious fact, the exact relation of which is yet far from clear. It is that if two top-knotted birds are matched, the young, instead of having very fine top-knots, are generally bald, or even have a wound on their heads. He suggests that it would appear as if the top-knot were due to some morbid condition which is increased to- an injurious degree when two birds in this state are paired. Of course the word "morbid," like many of its kind, is purely relative, and may mean either excess or diminution of a process. We have very little knowledge of the conditions which govern growth, but we are certain that vascular supply is an essential, and that the regulation of the same by the vaso- motor system governs the various modifications of the process. Thus the spur of the cock in its normal position has a definitely regulated blood-supply, which might be, by accident, increased or diminished so as to induce disease of the spur. But if it be placed under circumstances where its blood-supply is independent of vaso-motor control, as in John Hunter's experiments of engrafting it into the comb of the cock, its growth may be unlimited. Dr. Stirling has recently shown that this may be the case in organs which are not dislocated, as when the sympathetic is divided in the neck of a rabbit ; in course of time the ear increases very much in size, and the temperature is permanently exalted. In the case of the chicks of two top-knotted canaries, the baldness and wound may be really the result of an excess of blood-supply, just as the original top-knot must arise, and it would be extremely interesting to see what the interbreeding of birds so produced would lead to.

Mr. Darwin has studied canaries so closely, that he can tell us that they differ much in disposition and character, but that is a conclusion to which we think all who keep a number of animals of any one kind will come to. Even guinea-pigs, whose character is about the most insipid of all pets, vary considerably in disposition and tastes, and the degree of variation may be noticed to increase in proportion to the intelligence of the kind of animal observed. One of the most curious and at present one of the least explicable facts established by Mr. Darwin is the tendency of reversion to a wild state, or to very altered habits, by a mixture of race. Thus, if a hen belonging to a variety which does not incubate be crossed by another variety which has equally lost, by artificial selection, the tendency to brood, the product will be inclined to sit steadily on its eggs. A sow of the domesticated Chinese variety, crossed by an Alpine boar which had become remarkably tame, had young which were remarkably wild in confinement, and would not eat swill like common English pigs. Livingstone is quoted to the effect that "it is unaccountable why half-castes are so much more cruel than the Portuguese, but such undoubtedly is the case." And it was further remarked to the same missionary by a native that God made white men, and God made black men, but the Devil made half-castes." No explanation of this is, in the pre- sent state of our knowledge, possible ; but it gives an indisputable explanation of the impossibilities of keeping up mixed races, and it must also be offered as an explanation of the barbarities which we hear of as of frequent occurrence in such borderlands as at present extend across the whole continent of America, even un- der xanthochroic rule, and which we can only agree with our author in considering as reversion of types to a primitive state of savagery.

Equally incomprehensible, yet equally important and interest- ing, is the fact that in the case of certain crosses, one variety seems to have the tendency to perpetuate its peculiarities more than the other, and to this tendency Mr. Darwin gives the term of " prepotency in transmission." This has been seen to be re- markably the case in certain breeds of cattle, as the short-horn, and in the special instances of brood-mares and greyhounds. In certain human races this is markedly the case. Thus the child- ren of Frenchmen or Portuguese with East-Indian and Chinese mothers have, with the exception of the pigmentary develop- ment, the European characters preponderating strongly over the Asiatic, and are usually beautiful ; whilst the half-castes of Eng- lishmen and Germans have a tendency to show the Eastern blood rather than the Western, and are generally hideously ugly. In individual families this is also often to be remarked, and in the case of musical genius it is seen in very striking instances. Thus, of our great musical composers, the majority will be found to have had fathers who were noted musicians, but we have failed to find one instance in which the gift seems to have been trans- mitted from the female line. The children of Jews and Saxons seldom exhibit the Hebrew features with any prominence, especi- ally in childhood. Peculiarities of one sex are also apt to be continued in one sex, to the complete exclusion of the other, even when these peculiarities may be of a perfectly general character.

This is well seen in the case of certain diseases, as in a case quoted. from Dr. Sedgwick, in which four brothers suffered almost every week from severe head-aches, from which also their father, paternal uncles, paternal grandfather, and grand-uncles all suffer, yet all the female members of the family escaped. We also know of a.

case where all the women of a family suffer from nettle-rash if they eat strawberries, yet the males may eat them with impunity.

That marvellous tendency to variation which every animal dis- plays in every character forms the foundation upon which the- great factor of selection is brought to bear, either by natural agencies or by the conventional selection of man. Mr. Darwin's- writing on this subject may be said to be his best and most im- portant, though it is really difficult in a book like this to speak more highly of any oue part than of the whole.

Of artificial selection we need say no more here, but on the process of the evolution of new characters by natural selection, a

word or two may be said on a certain want of definition in the

of a term for which we are indebted to Mr. Herbert Spencer,. and which has now obtained a very extended use, so much so- that Mr. Darwin uses it as an equivalent for " natural selection.' The term in question, "survival of the fittest," is one which is.

far from expressing the whole of the steps of the process of natural selection, even as far as they are known to us ; and in- stead of having a universal application, as it has now in the mind of most writers, it seems to us that it should have one of a more restricted kind. When we use the superlative term "fittest," we obviously mean a limited number out of a multitude ; and when we speak of the "survival of the fittest," we infer the destruction of the majority by reason of their want of fitness. When, on the contrary, we say that certain animals survive " by reason of their fitness," we refer to the destruction of the few and the survival of the many ; and it is self-evident that these two conditions are quite different steps of the one process of evolution. To

explain this by example, let us suppose that a breed of sheep is introduced into a district cut up by ditches, and that to get their

food it shall be necessary for the sheep to be able to jump over these ditches. Suppose that the majority of these sheep have limbs only fourteen inches long, but that a few have a length of fifteen, sixteen, or even seventeen inches, and that these only can jump

the ditches. It is evident that the majority will perish by lack of fitness, and that the minority will survive by reason of their fitness.

But if the ditches are all exactly equal, and the sheep with limbs fifteen inches long are able to cross them, those with a length of limb seventeen inches will have no advantage, so that it is clear that this would not be a case of survival of the fittest, but one of survival by fitness. So again, if the sheep remained under the same condition, every year all who had limbs under the necessary length would perish, and thus the minimum limb-length would be maintained. This part of the process secures permanency of structure which has been evolved, but it in no way introduces anything new. But, on the other hand, suppose that one or two ditches were widened, so that only the seventeen-inch-legged sheep could cross, they would have a manifest advantage over all, and would have an increased chance of survival, by being the fittest. This would induce a further change of structure, and ks manifestly a wholly different step. "Survival of the fittest" if,