ADAPTATION TO VITAL CONDITIONS 147 



of any species with sufficient uniformity and regularity to 

 produce the numerous varieties wMch we see in the organic 

 world to-day, unless it be the environing conditions. But it 

 is evident that, although food and chmate play a role in the 

 production of new varieties, they could not possibly have effected 

 all those minute changes in the structure and function of 

 organisms, which are undoubtedly adaptations to the vital 

 conditions. When we consider, for iastance, the case of the 

 whale, or of the different bird species, or of the luminous organs 

 of deep-sea fishes — to mention only a few — it is difficult to see 

 how so far-sighted a biologist as Delage can reject the theory of 

 iadividual variation as the primary cause of the evolution of 

 species ; and go so far as to declare, with Lamarck, that food, 

 climate, use, and disuse, are the sole causes of such evolution. 



variation had set in at the time of separation, and, no condition having 

 since arisen capable of determining a variation, amixis alone has been 

 insufficient to effect one. 



That isolation or amixis has been a potent factor in the origin of new 

 species is sufficiently shoTvn by the figures quoted in this note, for aU 

 endemic species are, as their name indicates, due to isolation. Romanes 

 has used the term " isolation " in a more general sense, to include all the 

 various -n-ays in which the range of intercrossing is narrowed among 

 variants. Prominent among these is what we may term "sexual" 

 isolation, or the physiological hindrance to intercrossing among such 

 variants. It is evident that in the struggle for existence favoiirable 

 variants are selected at the expense of the original non-varying forms ; 

 that the latter, becoming ever less and less numerous, are eventually 

 suppressed ; and that the favoiu-able variants, unless they were " isolated " 

 from the parent stock, could never become permanent. This organic 

 progress is impossible without some form of isolation. The whole efficacy 

 of natural selection depends on a certain degree of " isolation." But this 

 more general " isolation," as indicated by Romanes, is not to be confused 

 with spatial isolation on a separate area ; and it is in this sense that Darwin 

 and Weismann use the term. 



The fewer species there are in possession of an island, the greater are 

 the chances of variation in an emigrant ; for the more space and food there 

 is, the better is it able to multiply. The more the descendants multiply, 

 the more they distribute themselves over the virgin territory, and the 

 more likelihood there is of numerous variations setting in, and of numerous 

 species being originated. This is probably the reason of the great number 

 of endemic species of snails on the Sandwich Islands. 



10—2 



