288 THE EVOLUTION THEORY 



varying in a particular direction occurred in the majority of its ids, 

 these would, as a consequence of persistent germinal selection, go on 

 varying gradually until the externally visible variation appeared. 

 This would not have appeared at all if the animal concerned had 

 remained in the original habitat of its species, for there it would 

 have been surrounded by normal germ-plasms, and its direct 

 descendants, even if they had been as favourably situated for the 

 origin of variations as we have assumed, would not have reproduced 

 only among themselves, and therefore even in the next generation the 

 number of Dv ids would have diminished. 



Obviously it is to a certain extent a matter of chance whether in 

 the isolated descendants the variation or the normal form remains the 

 victor, for it depends on the number of Dv ids originally present in 

 the fertilized eggs, then on the chances of reducing divisions, and 

 finally on the chance which brings together for pairing individuals in 

 which the similarly varied ids preponderate. The probability of the 

 conquest of the variation will depend in the main on the strength of 

 the majority of the varied ids in the fertilized eggs of the parents ; 

 if this be an overwhelming majority, then the chances of favourable 

 reducing divisions and pairings will also be great. The origin of 

 a pure amixia variety will thus depend upon the fact that the same 

 variational tendency Dv was present in a large number of the ids of 

 the ancestral germ-plasm. We need not wonder therefore that of the 

 numerous diurnal butterflies of Corsica and Sardinia only eight have 

 developed into endemic, probably ' amiktic,' varieties. 



But since we know that so many species in oceanic islands and 

 other isolated regions are endemic or autochthonous, i.e. of local origin, 

 there must obviously be some other factor in their evolution in 

 addition to the mere prevention of crossing with unvaried individuals 

 of the same species. The variational tendencies which have arisen in 

 the germ-plasm through germinal selection may — as we have already 

 seen — gain the ascendancy in various ways; first, by being favoured by 

 the climatic influences, then by being taken under the protection of per- 

 sonal selection, whether in the form of natural or of sexual selection. 



As the inhabitants of insular areas are not infrequently subject 

 to special climatic conditions, we may assume at the outset that many 

 of the ' endemic ' species are climatic varieties, but in many cases this 

 explanation is insufficient. For instance, special local forms of mock- 

 ing-bird live on several of the Galapagos Islands, but this cannot 

 depend upon diflferences of climate, for the islands are only a few 

 kilometres apart, and resemble one another as regards the conditions 

 of life which they present. But as the differences between these local 



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