52 THE; POUR SEGREGATIVE PRINCIPLES. 



manes says of infertility between varieties of the same species : ' ' For 

 the sake of convenience, and in order to preserve analogies with 

 already existing terms, I will call this principle physiological selection 

 or segregation of the fit."* Since the publication of his essay on 

 Physiological Selection in 1886, and of my papers on Divergent Evo- 

 lution and Intensive Segregation in 1887 and 1889, isolation has by 

 general consent come to mean the prevention of free-crossing between 

 groups existing at the same time. In accordance with this usage, in 

 Darwin and After Darwin, Romanes often substitutes physiological 

 isolation for physiological selection, which is a great gain. When, 

 however, he gives a precise definition of isolation, he extends its 

 meaning so as to include the prevention of crossing between those 

 members of the group who succeed in living and propagating and 

 those who die without. propagating. This definition of isolation 

 makes it include natural selection as one of its many forms. (See 

 Darwin and After Darwin, Part III, pp. 9, 10.) I recognize most 

 fully the importance of keeping in mind the fact that natural selec- 

 tion would have no power to transform species if it did not prevent 

 the crossing of the fit with the unfit; but I think the relation of the 

 different factors can be best presented, first, by restricting the term 

 " selection " to the influences that determine the survival (that is, the 

 continued propagation) of the fit innate variations of any given 

 group, and the elimination (that is, the disappearance) pf the unfit, 

 thus preventing the crossing of the fit with the unfit; second, by 

 restricting the term isolation to the prevention of free crossing be- 

 tween groups existing at the same time; and third, by showing how 

 these two principles cooperate in producing racial segregation of the 

 fit which is the essence of racial evolution. The conception of racial 

 evolution which I thus expound is much the same as that presented 

 by Romanes; but, if I mistake not, the meanings which I attach to 

 the different terms are in better accord with general usage. I also 

 attempt a wider problem, in that I now add to my exposition of 

 racial segregations and amalgamations a similar analysis of habitudi- 

 nal segregations and amalgamations, with the special purpose of 

 bringing clearly into view the action and reaction between the two 

 spheres of evolution. I think that even the most conservative biolo- 

 gists are coming to recognize with Mr. Headley that the racial evolu- 

 tion of the higher animals, and especially of man, is guided by their 

 social evolution (or by their progress, as he would put it), while many 

 already agree with him in his statement that "If natural selection 

 works without isolation, only monotypic evolution can result. "f 



* Journal Linnean Society, Zoology, Vol. XIX, p. 354. 

 f See Problems of Evolution, pp. 128, 175. 



