CHAPTER VII 



NATURAL SELECTION 



In this chapter we propose to examine as fully as possible the 

 validity of the theory of Natural Selection in so far as it 

 depends upon zoological evidence. We believe that a final 

 verdict on the efficacy of selection may be arrived at on zoo- 

 logical evidence and that there is no special category of botanical 

 data that is of crucial importance in determining the value of 

 this doctrine. 



In the seventy-six years that have elapsed since its first 

 announcement the main framework of this theory has remained 

 unchanged. It has been rejected by many and held by others 

 to have a less universal application than was originally believed. 

 We have obtained a clearer insight into the various natural 

 processes involved and a wider knowledge of the historical 

 facts of evolutionary change. But no material alteration of 

 the basic principles has been introduced and, for those who 

 subscribe to its tenets, it stands very much as it did when it 

 was first announced. Nevertheless, the volume of evidence 

 that may be produced both to support and to undermine it 

 has expanded and it is not inaccurate to say that the accumu- 

 lation of data on the various issues involved has outrun the 

 synthetic and comprehensive treatment of the subject. It 

 is therefore desirable at the offset to indicate what kind of 

 evidence is now available and to what degree of completeness 

 the field of inquiry has been covered. 



i. Darwin's Statement of the Evidence.— We may take 

 the evidence as presented in ' The Origin of Species ' (Darwin, 

 1884) as the chief demonstration by Darwin of the efficacy of 

 Natural Selection. In his letters and other works there is a 

 considerable mass of corroborative evidence and reasoning, 

 but the actual marshalling of the evidence for the operation 



