Opinions on Isolation. 125 



criticizing. For all that his argument goes to prove 

 is, first, the necessity for some form of isolation if 

 the overwhelming effects of intercrossing are to be 

 obviated ; and, secondly, the manifest consequence 

 that where the psychological form is unavailable (as 

 in many of the lower animals and in all plants), 

 some other form must be present if divergent evolu- 

 tion is taking place on a common area. 



Seeing that so much misunderstanding has been 

 shown with reference to my views on " the swamp- 

 ing effects of intercrossing," and seeing also that 

 this misunderstanding extends quite as much to Mr. 

 Gulick's views as to my own, I will here supply 

 brief extracts from both our original papers, for the 

 double purpose of showing our complete agreement, 

 and of leaving it to be judged whether we can 

 fairly be held responsible for the misunderstanding 

 in question. After having supplied these quotations, 

 I will conclude this historical sketch by considering 

 what Mr. Wallace has said in reply to the views 

 therein presented. I will transcribe but a single 

 passage from our papers, beginning with my own. 



Any theory of the origin of species in the way of descent must 

 be prepared with an answer to the question, Why have species 

 multiplied? How is it that, in the course of evolution, species 

 have not simply become transmuted in linear series instead of 

 ramifying into branches ? This question Mr. Darwin seeks to 

 answer " from the simple circumstance that the more diversified 

 the descendants from any one species becomes in structure, 

 constitution, and habits, by so much will they be better enabled 

 to seize on many and widely diversified places in the economy 

 of nature, and so be enabled to increase in numbers." And he 

 proceeds to illustrate this principle by means of a diagram, 



