238 



THE AMERICAN NATURALIST 



[Vol. XLI 



But this aside, the indications seem to me to be that a good many 

 instances sustaining the notion of mutative origin will be found 

 among plants. It is not to be expected that the number of such 

 cases will be relatively large. On the assumption of periodic 

 Mutation as the origin of species, with competition between 

 associated mutants and the survival of those mutants best fitted 

 for existence in the original habitat, and the spread of mutants 

 with new capacities into areas or habitats not open to the parental 

 species, we should expect to find as a rule a single species occupy- 

 ing a given territory or ecological footing, and related species in 

 separate, neighboring areas or habitats; though it is evident 

 that mutants instantly endowed by Mutation with physiological 

 or chronal isolation might continue to exist side by side with the 

 parental species or with sister mutants if there were no active 

 vegetative competition between the associated stocks. As a 

 matter of fact, in many species of plants competition for sub- 

 sistence between individuals of the same parentage is practically 

 absent. Unification of congenital mutants may be brought about 

 by continued interbreeding. This would eventually destroy the 

 geographic evidence of Mutation in any given case. But in such 

 amalgamation the effects of Mutation may not be destroyed ; for 

 new characters may during amalgamation be perpetuated in full 

 force. It is single characters, rather than constellations of charac- 

 ters, with which the Mutation Theory is primarily concerned 

 The number of cases of association of closely related species 

 resembling recent mutants, in proportion to the number of cases 

 of geographic or topographic segregation of closely related species 

 would depend upon the balance betv^^een the activity of Mutation 

 on the one hand and the operation of the forces tending to isolate 

 or to amalgamate the products of Mutation on the other. If 

 mutative periods are far apart in most species — and stabihty 

 of the organic world may preclude great frequence — while the 

 segregating or amalgamating powers are constantly at work, 

 then the occurrence of the social condition indicative of Mutation 

 may be expected to be relatively infrequent. 



In order to use geographical evidence effectively against the 

 Mutation Theory, its opponents must show that the social con- 

 dition of closely related forms is, to use President Jordan's words. 



