36 THE EVOLUTION OF NATURAL SPECIES. 
The principle underlying both classes of cases is that free intergen- 
eration insures unity of type, and that the prevention of free com- 
munication and of free crossing prevents the operation of reflexive 
selection* between the isolated groups, and also opens the way for 
diversity in the use of the environment, and so leads to diversity in 
other forms of selection. Isolation, therefore, codperating with the 
power of variation and with the principle of selection of other forms 
than natural selection, goes far toward explaining the phenomena 
which we have been considering, and which are essentially the 
same as those which Mr. Bateson has cited as being in pressing need 
of explanation. Discontinuous variation explains the lack of inter- 
mediate forms in certain cases, but it isnot the necessary explanation 
in every case. 
The related subject of mutation is briefly considered in Chapter V, 
at the end of section III. Those interested in the subject of the dis- 
continuity of closely related species will find an interesting summary 
of the facts and interpretations in ‘‘The Method of Evolution,” by 
Professor Conn, pages 35, 115 to 139, 359. 
* Reflexive selection is described in Appendix II, I, 8, (3), as ‘‘depending on 
the relations of the members of a species to each other.’’ The most famil- 
iar forms of reflexive selection are sexual and social selection. It is, of course, 
manifest to every one that two completely separated groups of the same species 
can have no influence over each other through sexual and social selection. Other 
forms of reflexive selection are considered in Chapter VI; alsothe fact that there 
are endonomic forms of environal selection. 
