70 THE PROBLEMS OF EVOLUTION 



by distinct varieties on each of several of the 

 larger islands" and lists for Camponotus macilentus 

 ten varieties, including the typical form, occurring 

 on eleven islands. In this remarkable series only 

 two varieties occur on two islands and only one 

 island has two varieties. ^^ Conclusions regarding 

 the origin of the archipelago and its fauna are by 

 no means fixed, but it is at least probable that the 

 islands were long ago isolated from the Americas as 

 a single mass of land which later gave rise to the 

 separate islands of the existing group, thus isolat- 

 ing portions of the parent species. Such a history 

 would account nicely for the varieties inhabiting 

 oceanic islands. 



The types of selection emphasized by Darwin 

 also tend to preserve a limited range of the he- 

 reditary characters of species by eliminating the 

 individuals which do not possess them. We cannot 

 doubt the reality of these processes, however much 

 they may have been overemphasized in the past. 

 Both natural selection and sexual selection are 

 wholly logical in theory. Like many other prob- 

 lems of evolution, particular cases may be obscure 

 in detail, but the principles involved are too well 

 established to be discarded as ineffective. Useful- 

 ness is the keynote of adaptation. We cannot 

 doubt that organs occur in various hereditary de- 

 grees of development, nor that the individuals 



" Proc. Cal. Acad. Sci. (4), II (2), p. 263, 1919. 



