30 MODERN IDEAS OF EVOLUTION 



must have been operative, although many able Dar 

 winians, like Weismann and Wallace, profess to regard 

 natural selection as the sole operative cause. The 

 influence of an innate tendency to vary has been 

 claimed by some, as if in the original creation of 

 living beings they had been so wound up as to 

 go first in one direction and then in another, 

 without any external cause, or when acted upon 

 by varying causes. The influence of favourable 

 conditions and room for expansion has been alleged 

 by others, in accordance with the old view of Lamarck. 

 The tendency of some lower animals, under unfavour 

 able conditions, to become reproductive before they 

 have attained to full maturity, while more favourable 

 circumstances elevate the standing to which the 

 animal attains before producing young, is also a con 

 sideration which, under the name of reproductive 

 acceleration or retardation, has attracted some atten 

 tion. Various causes of abrupt or sudden change 

 have also been invoked, as, for instance, those obscure 

 agencies which determine the appearance of monstro 

 sities or varietal forms among domesticated animals. 

 The question of efficient cause has thus become 

 very complicated, and the only points on which all 

 are united are the possibility of varieties or races in 

 some way overleaping the bounds of specific fixity 

 and becoming new species, and the further doctrine 

 that changes acquired in any way may become per 

 manent as an inheritance in such new species. This 



