70 Darwin, and after Darwin. 



adaptations which they involve differ from those where 

 no manufacture, so to speak, of special machinery is 

 required. Thus, it is easy to understand how natural 

 selection alone is capable of gradually accumulating 

 congenital variations in the direction of protective 

 colouring ; of mimicry ; of general size, form, mutual 

 correlation of parts as connected with superior strength, 

 fleetness, agility, &c.; of greater or less development 

 of particular parts, such as legs, wings, tails, &c. For 

 in all such cases the adaptation which is in process of 

 accumulation is, from its very commencement and 

 throughout each of its subsequent stages, of use in 

 the struggle for existence. And inasmuch as all the 

 individuals of each successive generation vary round 

 the specific mean which characterized the preceding 

 generation, there will always be a sufficient number of 

 individuals which present congenital variations of the 

 kind required for natural selection to seize upon, 

 without danger of their being swamped by free in- 

 tercrossing as Mr. Wallace has very ably shown in 

 his Darwinism. But this law of averages can apply 

 only to cases where single structures or a single 

 group of correlated structures are already present, 

 and already varying round a specific mean* The case 

 is quite different where a co-ordination of structures is 

 required for the performance of 2*. previously non-existent 

 reflex action. For some, at least, of these structures 

 must be new, as must also be the function which all of 

 them first conspire to perform. Therefore, neither the 

 new elements of structure, nor the new combination of 

 structures, can have been previously given as varying 

 round a specific mean. On the contrary, a very 

 definite piece of machinery, consisting of many co- 



