72 Darwin, and after Darwin. 



In the second place, adaptations due to organic 

 machineries of this kind differ in another all-important 

 respect from those due to a summation of adaptive 

 characters which are already present and already- 

 varying round a specific mean. The latter depend for 

 their summation upon the fact — not merely, as just 

 stated, that they are already present, already varying 

 round a specific mean, and therefore owe their pro- 

 gressive evolution to free intercrossing, but also — that 

 they admit of very different degrees of adaptation. It 

 is only because the degree of adaptation in generation 

 B is superior to that in generation A that gradual 

 improvement in respect of adaptation is here possible. 

 In the case of protective resemblance, for example, 

 a very imperfect and merely accidental resemblance 

 to a leaf, to another insect. &c., may at the first start 

 have conferred a sufficient degree of adaptive imitation 

 to count for something in the struggle for life ; and, if 

 so, the basis would be given for a progressive building 

 up by natural selection of structures and colours 

 in ever-advancing degrees of adaptive resemblance. 

 There is here no necessity to suppose — nor in point 

 of fact is it ever supposed, since the supposition 

 would involve nothing short of a miracle — ^that such 

 extreme perfection in this respect as we now so fre- 

 quently admire has originated suddenly in a single 

 generation, as a collective variation of a congenital 

 kind affecting simultaneously a large proportional 

 number of individuals. But in the case of a reflex 

 mechanism^which may involve even greater marvels 

 of adaptive adjustment, and all the parts of which 

 must occur in the same individuals to be of any 

 use — it is necessary to suppose some such sudden 



