CONCLUSIONS 371 



form) postulates that the evolutionary process is unitary, and 

 that not only are groups formed by the multiplication of single 

 variants having survival value, but also that such divergences 

 are amplified to produce adaptations (both specialisations 

 and organisation). It has been customary to admit that 

 certain ancillary processes are operative (isolation, correlation), 

 but the importance of these, as active principles, is sub- 

 ordinate to selection. The evidence for the efficacy of selection 

 is summarised in Chapter VII. It will be seen there that 

 (a) it is very doubtful whether we have enough evidence of 

 the right sort to form a judgment ; (b) the direct evidence 

 is negligible ; and (c) the bulk of the circumstantial evidence 

 is inadequate, although in some instances we are impelled to 

 recognise that the action of selection is likely, if not proved. 

 Conversely, there is a good deal of evidence that suggests that 

 races and species arise independently of the survival value of 

 their characters, unless we are prepared to make a very large 

 appeal to ignorance. Apart from the strong theoretical case — 

 which we do not regard as evidential — presented for Natural 

 Selection as an agency adequate to account for the spread of 

 new characters, it seems that the verdict must turn on the 

 amount of weight we are prepared to allow to the various 

 pieces of circumstantial evidence (mimicry, Cuckoo's eggs, 

 etc.). We feel that these are by no means negligible and, in 

 default of very convincing alternative explanations, they must 

 remain as testimony that selection may be operative. Selec- 

 tion must therefore be retained as a likely factor. If this is 

 admitted, it is only fair to ask : if the activity of Natural Selec- 

 tion is admitted as probable in some cases, may it not be more 

 widely operative ? Is it likely that such a principle should 

 have only a partial or particular efficacy ? Such questions 

 plainly cannot be answered except on grounds so general as to 

 be devoid of value. There is no a priori reason for considering 

 that Natural Selection must have a universal activity, even if 

 its efficacy is demonstrated in particular cases. 



We attach considerable importance to the facts assembled 

 in Chapter VII which suggest that the divergence of races and 

 species is not influenced by selection. It has been suggested 

 (p. 251) that, if mimetic resemblances are shown to be produced 

 by selection, it involves a strong presupposition that specific 

 divergences of the same order must be produced by this 



