148 DARWINISM chap. 



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

 lengthy pajDer, presented to the Linnean Society last year, on 

 "Divergent Evolution through 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'vving 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 generatioii as long as the individuals of the two 

 groups are kept from intercrossing." ■'■ 



It need hardly be said that the views of Mr. Darwin and 

 myself are inconsistent with the notion that, if the environment 

 Avere 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 Avhat 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, we are so 

 I profoundly ignorant. The exact proportions of the various 

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

 l>ird, the peculiarities of more or less exposure to sunshine 

 or to Avind at certain critical epochs, and other slight 

 ' diff"erences Avhich 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, Avhich natural 

 selection will bring about. All Ave knoAv of the facts of 

 variation leads us to believe that, Avithout this action of 

 natural selection, there Avould be produced over the whole area 

 a series of inconstant varieties mingled together, not a distinct 

 seOTCiration of forms each confined to its OAvn limited area. 



Mr. Darwin has shoAvn that, in the distribution and 

 modification of species, the biological is of more importance 

 than the physical environment, the struggle Avith other 

 organisms beins; often more severe than that Avith the forces 

 of nature. This is particularly evident in the case of plants, 

 many of Avhich, Avhen protected from competition, thrive in a 

 ' Jownal of the Linnean Society, Zoology, vol. xx. p. 215. 



