NATURAL SELECTION 309 



the basal, the other for the apical part of the antenna. 

 In some species only a part of the antenna can be with- 

 drawn into the scrobe. In many beetles similar grooves are 

 developed : in the Elateridae, for instance, but here they are 

 situated on the under-side of the head and thorax. In the 

 Byrrhidae each segment of the legs is grooved to contain the 

 following one, so that the legs are almost invisible when re- 

 tracted. Another type of co-adaptation is seen in the raptorial 

 foreleg found in many groups of insects {Mantis, Mantispa, 

 Phymatidae, etc.). Here the curved and apically spurred fore 

 tibia can be adpressed to the strong, multispinose fore femora. 

 In all these co-adaptations the final form of the structure 

 appears, at least very plausibly, to be adaptive, but it is very 

 difficult to imagine their origin under the influence of Natural 

 Selection. The early stages in the development of co-adapted 

 parts 1 appear to be unsuitable for the purpose to which the 

 finished structure is put, while in many cases the co-adaptation 

 could be adaptive in the early stages of its evolution only if 

 a number of independent variations occurred simultaneously, 

 for the essence of such a structure is the co-operation between 

 different parts. Cuenot, Wheeler and Corset all agree that 

 many co-adaptations cannot be explained on the selection 

 theory, though no other explanation can as yet be put forward. 

 We shall return to this question in our discussion of ortho- 

 genesis. 



Summary 



A preliminary examination of the data reveals that most 

 workers have considered the deductive consequence of the 

 Natural Selection theory rather than provided direct evidence 

 for it. Most of the facts recorded by Darwin in ' The Origin ' 

 are evidence for evolution as opposed to ' special creation.' 

 Only a minor part of the work deals at all directly with evidence 

 for the theory of Natural Selection, which appears scarcely 

 to have been distinguished in Darwin's mind from the more 

 general proposition that species have arisen by descent with 

 modification. The problem has been somewhat clarified by 

 recent advances in our knowledge, but it is still on analysis of 

 the consequences of selection rather than on the demonstration 



1 The special case of the co-adaptation of the male and female genitalia of a 

 species is considered earlier (p. 151). 



