INTENSTTE SE&REGATrOIf. 351 



of the species thrive only where the shade is dense and the atmo- 

 sphere laden with dampness a large portion of each month. 



The student who starts with the assumption that divergent 

 varieties and species arise only through exposure to different 

 environments, will expect that these groves, at least those on the 

 same side of the mountain-range, will be occupied by the same 

 species. Having found one set of species in a given valley, when 

 he comes to a valley ten miles distant possessing the same con- 

 ditions of soil, rainfall, vegetation, and shade, where the birds, 

 reptiles, and insects are the same, where the mice and ants, tlieir 

 only known enemies, are the same, he naturally looks on the 

 leaves and branches of the familiar trees for the snails he has 

 found in similar stations not far distant ; but what is his surprise 

 to find only different species, all allied to, but quite distinct from, 

 those he has previously known ! Twenty miles from the first 

 valley he renews his investigations, finding the forms of all the 

 diff"erent groups still more divergent, though all the conditions 

 of the environment are, so far as he can observe, the same. 



He finally perceives that he must either assume that there are 

 occult influences in the environment varying with progressive 

 force with each successive mile, or he must give up the theory 

 that the cause of this divergence is exposure to dift'erent envi- 

 ronments. 



3. When the environment is the same in two districts occupied 

 hx] allied species or varieties, it is evident that the differences 

 that distinguish the latter cannot he advantageous, even 

 though their differences include strongly contrasted habits. 



For in order that these difi'erences should be advantageous, 

 it is necessary not only that they should relate to the performance 

 of vital f unctioQs, and therefore be differences of adaptation, but 

 it is necessary that these difBerences of adaptation should relate 

 to differences in the environment, so that the forms would be at 

 some disadvantage if they should exchange districts. Adapta- 

 tional specific dift'erences are not always advantageous, ard in 

 such cases the divergence cannot be primarily attributed to 

 diversity in the action of natural selection in the diflerent 

 districts. Under the protection of Isolation, diversity of natural 

 selection may arise which helps in producing divergence ; but 

 when the environments are the same, the divergence is in no 



