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 indhndiials to be of any 

 use it is necessary to suppose some such sudden 



