tribes of men, this selection is no doubt carried on unconsciously, but among the breeders of cattle, or among pigeon-fanciers, it is a strictly scientific process, carried on consciously and deliberately, and according to rules, which are none the less fixed that they are largely 'rules of thumb.'

Those who wish to understand this most interesting subject in all its bearings must turn to the pages of the 'Origin of Species,' where it is fully treated by the hand of the master. All that need be done here is to say one word as to the relation between the known facts of'artificial selection' on the one hand and the theory of the origin of wild species by 'natural selection' on the other hand. If it be admitted, namely, that our numerous varieties of domesticated animals owe their peculiarities to the 'selection' exercised by man during the comparatively brief period during which he has existed upon the earth, it is not unreasonable to suppose that 'natural selection,' operating through an infinitely longer period, and by methods much more subtle and far-reaching, has produced the different wild 'species' of animals by modifications of one or more aboriginal types. The unquestionable facts, therefore, as to the production of our domesticated breeds of animals from wild species by means of 'artificial selection,' afford a strong presumption in favour of the theory that our existing wild species have been produced by the modification of pre-existing wild species through the operation of 'natural selection.'

THE THEORY OF NATURAL SELECTION

(CONTINUED).

HAVING now given the briefest possible sketch of the Theory of Natural Selection, as expounded by Mr Darwin, it may be well to notice, with equal brevity, the leading objections which have been urged against this theory by various naturalists, and notably by Mr Mivart.* It may also be as well to enumerate shortly the chief general grounds upon which naturalists base the now generally accepted belief that species have been produced from preexisting species by the action of some law of evolution, apart from the question of the method or methods in which this law operates.

Numerous difficulties admittedly have to be met, if we attempt to apply the theory of natural selection (even when combined with what Darwin has called 'sexual selection') as the sole principle involved in the production of 'species.' Many of these difficulties are of a special nature, affecting special cases only, and they need no discussion here. It is possible that many of these special * The Genesis of Species, by St George Mivart, 1871.

difficulties may disappear in the light of wider knowledge. There are, however, certain general difficulties which demand a moment's consideration, as indicating that though we admit the action of 'natural selection' to the full, we must nevertheless look beyond and outside this for the complete explanation of the existence and origin of species. The general difficulties in question were perfectly recognised by Mr Darwin, and have been met by him, as far as it is at present possible to meet them. The principal are the following: *

(1) One of the most general, and certainly one of the most serious of the difficulties in the way of the theory of natural selection is 'the uselessness of many organs in their incipient stage.' Hosts of structures (such as the milk-glands of the Quadrupeds, or the whalebone plates in the mouth of the Whalebone Whales) are exceedingly useful to the animal when perfectly developed; but it is inconceivable that they could have benefited the animal when first they began to be developed. According to the theory of the evolution of species in general, and the theory of natural selection in particular, milk-glands did not exist in the animal forms out of which the class of the Mammals was evolved, nor did baleen-plates exist in the ancestors of the Whalebone Whales. There must, therefore, have been a time when milk-glands and baleen-plates respectively first came into existence, and it is impossible to suppose that they were suddenly produced in complete structural and functional perfection as we now see them. On the contrary, they must, to begin with, have been mere

* An excellent résumé of these objections is given by Mr Pascoe in his Notes on Natural Selection and the Origin of Species, 1884.

rudimentary structures, functionally useless, and it can only have been in the course of development during many successive generations, that they assumed their present perfection. Now there is absolutely no evidence to show that the fine beginnings of structures can be useful or profitable to the animal possessing them. They may be harmless, but that is all that can be said. It is, however, the very essence of the theory of natural selection, that the law of the struggle for existence is powerless to preserve or intensify any structures except such as are useful to the individual. The fact that a structure may be useful to the race is not enough, as final causes or ends are wholly excluded from the theory of natural selection. Upon the whole, the difficulty of accounting for the preservation of incipient organs and structures by the action of natural selection appears to constitute the most formidable of the arguments which have been urged against Mr Darwin's views; since it is a general difficulty, and strikes at the very root of the theory of natural selection.

(2) A second general objection of great weight is that unless many individuals should be similarly and simultaneously modified,' there would be little chance of any useful variation which might have appeared in a species being ultimately preserved and handed down. Any new structure or organ, or any alteration in a pre-existing structure, must be slowly produced, and pass through an incipient stage. If, however, such a new structure, or alteration in an old structure, appeared, to begin with, in only one or two individuals of a species, it could hardly be preserved, as it would be 'lost by subsequent intercrossing with ordinary individuals.' But it is hardly

probable that any variation would simultaneously appear in many individuals of a species; and we have at any rate no evidence to show that this ever occurs.

(3) The theory of the origin of species by means of natural selection, in the third place, implies that the production of any given species from any pre-existing. species can only be effected by gradual modification, and therefore through the intervention of a long series of intermediate or transitional forms. Moreover, the transitional forms by which we should pass from a given species to the pre-existing species from which it was developed, must, on the theory of natural selection, be so closely related to one another as to render it difficult to distinguish them. In other words, if we had before us all the forms by which one species had been gradually converted into another, we should not have the slightest difficulty in recognising the distinctness of the individuals forming the extreme terms of the series; but the individuals standing between the extremes would pass into one another by such fine gradations as to render their separation almost or quite impossible. It seems also clear that, in the modification of any one species into any other, the total number of the individuals of intermediate or transitional form must greatly exceed the total number of individuals contained in the original species and the new species put together. Now, if all species of animals, living and extinct, have been produced by gradual modification from pre-existing species, we ought to find abundant evidence of the existence of the infinite number of transitional forms postulated by the theory of natural selection. In fact, as these transitional