220 EEV. J. T. GULICK ON DIYERGENT EYOLUTION 



find that the different branches are liable to be again subjected 

 to segregative influences, by which each branch is subdivided, 

 and in time differentiated into divergent forms that are not liable 

 to intercross in a state of nature. 



Now, as we have just pointed out, we know, from the funda- 

 mental laws of the organic world, that Cumulative Segregation 

 of this kind must produce Cumulative Divergence of Types. 



The Segregation that results from the natural causes enume- 

 rated in this paper is cumulative in two respects. In the first 

 place, every new form of segregation that now appears depends 

 on, and is superimposed upon, forms of Segregation that have 

 been previously induced ; for when Negative Segregation arises, 

 and the varieties of a species become less and less fertile with 

 each other, the complete infertility that has existed between them 

 and some other species does not disappear, nor does the Positive 

 Segregation (that is, the prevention of the consorting of the 

 species characterized by this mutual incapacity) cease. The 

 means by which the males and females of one species find each 

 other are not abrogated when the species falls into segregated 

 varieties. In the second place, whenever Segregation is directJy 

 produced by some quality of the organism, variations that possess 

 the endowment in a superior degree will have a larger share in 

 producing the segregated forms of the next generation, and 

 accordingly the Segregative endowment of the next generation 

 will be greater than that of the present generation ; and so with 

 each successive generation the segregation will become in- 

 creasingly complete. 



The principle of Cumulative Segregation, first in its inde- 

 pendent action, and still further when combined with the differ- 

 ent principles by which the divergence of the segregated branches 

 is intensified, gives a formal explanation of the ever-expanding 

 diversities of the organic world. It shows how varieties arise 

 and pass into species, how species pass into genera, genera into 

 families, families into orders, and orders into classes and the 

 higher divisions, as far as evolution by descent extends. It 

 brings to light the dependence of this whole process on the 

 influences that produce segregation ; and shows how these 

 influences, added to Variation, Heredity, and the other acknow- 

 ledged powers residing in organisms, must produce the phe- 

 nomena of divergent evolution. 



