52 THE FOUR 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) of 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 codperate 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.”’t 
* Journal Linnean Society, Zoology, Vol. XIX, p. 354. 
¢ See Problems of Evolution, pp. 128, 175. 
