June 15, 1900.] 



SCIENCE. 



929 



passing over this third possibility, let us con- 

 sider for the moment which of the two first 

 named alternatives is likely to have oc- 

 curred. 



According to the Darwinian theory of evo- 

 lution, one of the most important factors in 

 determining the modification of organisms 

 has been natural selection. Selection acts 

 by preserving certain favorable variations, 

 and allowing others less favorable to be 

 killed off in the struggle for existence. It 

 thus will come about that certain variations 

 will be gradually eliminated. Meanwhile 

 the variations of the selected organisms will 

 themselves be submitted to selection, and 

 certain of these will be in their turn elimi- 

 nated. In this way a group of organisms 

 becomes more and more closely adapted to 

 its surroundings; and unless new variations 

 make their appearance as the old unfavor- 

 able ones are eliminated, the variability of 

 the species will diminish as the result of se- 

 lection. Is it likely that new variations 

 will appear in the manner suggested ? To 

 answer this question we must turn to the 

 results obtained by human agency in the se- 

 lective breeding of animals. The experi- 

 ence of breeders is that continued selection 

 tends to produce a greater and greater 

 purity of stock, characterized by small vari- 

 ability, so that if the selective breeding is 

 carried too far, variation almost entirely 

 ceases, and there is little opportunity left 

 for the exercise of the breeder's art. "When 

 this condition has been arrived at, he is 

 obliged, if he wants to produce any further 

 modifications of his animals, to introduce 

 new blood — i. e., to bring in an individual 

 which has either been bred to a different 

 standard, or one in which the variability 

 has not been so completely extinguished. 



It would thus appear, and I think we are 

 justified in holding this view, at any rate 

 provisionally, that the result of continued 

 selection will be to diminish the variability 

 of a species ; and if carried far enough, to 



produce a race with so little variability, and 

 so closely adapted to its surroundings, that 

 the slightest alteration in the conditions of 

 life will cause extinction.* 



If selection tends to diminish the vari- 

 ability of a species, then it clearly follows 

 that as selection has been by hypothesis 

 the most important means of modifying or- 

 ganisms, variation must have been much 

 greater in past times than it is now. In 

 fact it must have been progressively greater 

 the farther we go back from the present time. 



The argument which I have just laid 

 before you points, if carried to its logical 

 conclusion — and I see no reason why it 

 should not be so carried — to the view that 

 at the first origin of life upon the earth the 

 variability of living matter consequent upon 

 the act of conjugation must have been of 

 enormous range ; in other words, it points 

 to the view that heredity was a much less 

 important phenomenon than it is at present. 

 Following out the same train of thought, we 

 are inevitably driven to the conclusion that 

 one of the most important results of the evo- 

 lutionary change has been the gradual in- 

 crease and perfection of heredity as a func- 

 tion of organisms and a gradual elimination 

 of variability. 



This view, if it can be established, is of 

 the utmost importance to our theoretical 

 conception of evolution, because it enables 

 us to bring our requirements as to time 

 within the limits granted by the physicists. 



*The expression extinction of species seems to be 

 used in two senses, ■which are generally confused. 

 Firstly, a species may become modified so that the 

 form with which we are familiar gradually gives 

 place to one or more forms which have been gradu- 

 ally produced by its modification. That is to say, a 

 character or series of characters becomes gradually 

 modified or lost in successive generations. This is 

 not really extinction, but development. Secondly, a 

 species may gradually lose its variability, and become 

 fixed in character. If the conditions then change, it 

 is unable to adapt itself to them, and becomes truly 

 extinct. In this case it leaves no descendants. "We 

 have to do with death, and not with devel pment. 



