INFLUENCE OP ISOLATION 143 



readaptation to its changed environment. The variation will 

 be readily persistent in proportion as the species in its new 

 surroundings remains isolated from the species whose habitat 

 it formerly shared. And likewise, if a single individual of a 

 species be transported into a new environment, any variation 

 which may set in will have a greater chance of becoming per- 

 manent, and of evolving into a new and well-defined species, 

 if this individual and its descendants are isolated from the parent 

 species. 



It may be stated, as a general law of evolution, that the 

 development and differentiation of species is promoted by iso- 

 lation. Suppose the normally fertilised female of a butterfly 

 to be transported by a bird from the mainland to an island on 

 which no single member of the species existed. The fact that 

 this butterfly could multiply without crossing with individuals 

 of the parent species on the mainland would not in itself be any 

 reason for the evolution of a new variety. But supposing this 

 butterfly to differ, in any insignificant feature of colouring, from 

 the parent species — as might, indeed, easily arise from germinal 

 selection — ^the very fact of its isolation from the parent species 

 will almost certainly ensure the preservation of this variation ; 

 for, supposing the latter to exist in a majority of ids in the germ- 

 plasm of the parent butterfly, it wiU be transmitted to a majority 

 of ids in the germ-plasm of the next generation ; and, as there 

 is no risk of amphimixis with individuals not possessing the 

 variation the latter is very likely to be maintained, and to become 

 eventually, through amphimixis, a characteristic of the species. 

 The reduction of the ids at maturation is a risk, but if the varia- 

 tion exists in a large majority of ids, this risk is considerably 

 lessened ; and in proportion as the variation obtains in an 

 ever greater number of individuals, the maturation reduction 

 loses its importance. Variations which develop into character- 

 istics of a species solely through isolation may be termed " varia- 

 tions effected through amixis." 



