196 APPENDIX II — INTENSIVE SEGREGATION. 



race is usually the result. In other words, balanced selection produces 

 stability of type, and unbalanced selection produces transformation of type. * 



In the Ught of this twofold law we see how there may be stringent 

 selection without transforming effect. In nearly every species there is 

 a constant struggle between the different forms of variation ; and as it 

 never happens that all the forms are equally successful, the process of 

 natural selection is always bearing in full force upon the species. If, 

 then, it could be shown that natural selection, wherever it exists, must 

 necessarily produce transformation, it would be impossible to resist 

 the conclusion that nearly every species is undergoing transformation 

 through this cause. But it is unbalanced rather than balanced selec- 

 tion that produces transformation. We also see that heredity tends 

 to make the most successful form the average form, and thus to con- 

 vert unbalanced into balanced selection. From this it follows that in 

 order that selection should produce continuous transformation, cov- 

 ering a wide range, it is necessary that the form of variation selected 

 should from time to time be changed. This may be expressed as the 

 principle of continuous transformation through successive changes in the 

 character of the selection. 



Though selection produces transformation only when it involves 

 the survival of other than typical forms, it is still very possible that 

 there are only a few species in which completely balanced selection 

 prevails for very many generations in succession. It is still certain 

 that long-continued independent selection gradually passes into diver- 

 sity of selection producing divergent evolution. 



(3) Though in more than one passage Darwin maintains that uni- 

 formity of external conditions involves uniformity of natural selec- 

 tion, and that isolation can have no effect in transforming a species if 

 physical conditions and surrounding organisms remain the same, 

 still I think that if the question had been distinctly brought before 

 him he would have admitted that exposure to a new or changed 

 environment was not a necessary condition for change in the char- 

 acter of sexual selection. Now, I think it can be shown that, besides 

 sexual selection, there are several forms of selection that depend upon 

 the relations of the members of one species to each other and that 

 may undergo change without the organism being exposed to a different 

 environment. 



Selection depending on the relations of the organism to the envi- 

 ronment I call environal selection, of which I find two kinds, namely, 



* The general law is here stated, without any attempt to explain why selection, 

 securing the exclusion of those falling below a certain standard, is necessary for 

 the preservation of that standard in the case of many characters (see par. (6) below) . 



