I'K EFACI 



In the presenl volume 1 have broughl together in one connected 

 presentation the chief results of tnv investigations concerning the 

 factors of organic evolution. Portions of my theory of diverge] 

 which were published in the Linnean Society's Journal arc- repro 

 duced in the Appendix, with careful revision; but the fullesl exposi 

 lion of the fact thai all evolution, as we now observe it, is divergent, 

 and that other factors besides natural selection are absolutely neces 

 sary both Tor the origin and the continuance of this divergence, is 

 given in the new chapters constituting the body of the volume. 

 These chapters have been written while considering the most recent 

 biological investigations bearing on the general theory of segregal ion. 

 The first four chapters of the volume arc- in l roductory, in t hat thev 

 present many facts of divergence and distribution in both natural and 

 domestic species, which remain complete enigmas till the forms of 

 racial and habitudinal segregation have been fully recognized. Chap- 

 ters V, VI, and VII present the fundamental laws of segregation, and 

 the interaction between the different classes of factors between 

 isolation and selection, between racial segregation and habitudinal 

 segregation, between autonomic factors and heteronomic factors. 

 In Chapter VI, § 11, 1 4— 1 7 (pp. 101-1 1 1), will be found a fuller exposi 

 lion than has been presented in any of my essays published by the 

 Unuean Society, of the tendency of certain combinations of partially 

 segregative endowments to become more intense in successive genera 

 lions. It is shown that this is especially the case when endowments, 

 tending toward the mating of like forms with each other, are reinforced 

 by varying degreesof mutual infertility and incompatibility between 

 unlike forms. Appendix II, >; IV, 3 (pp. 241-243), briefly indicates 

 several methods of constructing what I have called the permutation 

 triangle. It was first constructed in order to show that the sterility of 

 cross-unions between divergent forms (whether they lie varieties, 

 species, genera, or higher groups), would lead rapidly to the extinction 

 of most of these forms, if instincts and other endowments did not 

 facilitate the union of compatible forms. The table thus constructed 

 is found to be a concise presentation of certain classes of probabilities 

 that arise in the pairing of things by chance. 



