148 DARWINISM 



another on the same side an eqnal distance apart. In a very- 

 lengthy paper, i)resented to the Linnean Society last year, on 

 "Divergent Evohition tlnough Cumulative Segregation," Mr. 

 Gulick endeavours to work out his views into a complete 

 theory, the main point of which may perhaps be indicated by 

 the follo\Wng passage : " No two portions of a species possess 

 exactly the same average character, and the initial differences 

 are for ever reacting on the environment and on each other 

 in such a way as to ensure increasing divergence in each 

 successive generation as long as the individuals of the two 

 groups are kept from intercrossing."^ 



It need hardly be said that the views of ]\Ir. Darwin and 

 myself are inconsistent vnth. the notion that, if the environment 

 were absolutely similar for the two isolated portions of the 

 species, any such necessary and constant divergence would 

 take place. It is an error to assume that what seem to 

 us identical conditions are really identical to such small 

 and delicate organisms as these land molluscs, of whose 

 needs and difficulties at each successive stage of their existence, 

 from the freshly-laid egg up to the adult animal, Ave are so 

 profoundly ignorant. The exact proportions of the various 

 species of plants, the numbers of each kind of insect or of 

 bird, the peculiarities of more or less exj^osure to sunshine 

 or to wind at certain critical epochs, and other slight 

 differences which to us are absolutely immaterial and un- 

 recognisable, may be of the highest significance to these 

 humble creatures, and be quite sufficient to require some 

 slight adjustments of size, form, or colour, which natural 

 selection will bring about. All we know of the facts of 

 variation leads us to believe that, without this action of 

 natural selection, there would be produced over the whole area 

 a series of inconstant varieties mingled together, not a distinct 

 segregation of forms each confined to its own limited area. 



Mr. Darwin has shown that, in the distribution and 

 modification of species, the biological is of more importance 

 than the physical environment, the struggle with other 

 organisms being often more severe than that with the forces 

 of nature. This is particularly e\adent in the case of plants, 

 many of which, when protected from competition, thrive in a 

 ^ Journal of the Linnean Society, Zoology, vol. xx. p. 215. 



