2 THE VARIATION OF ANIMALS IN NATURE 



The facts of variation impressed themselves on the early 

 systematists and the collection and utilisation of such data are 

 a part of systematic zoology. The analysis of the vast body 

 of facts thus accumulated and the extraction of general prin- 

 ciples from them were, of course, stimulated by the work of 

 Darwin and Wallace and became important items in the 

 technique of evolutionary studies. Various aspects of the 

 problem have been dealt with in a number of synthetic 

 works : Bateson (1894), Woltereck (191 9), Philiptschenko 

 (1927), Rensch (1929). The origin of variation and its 

 hereditary distribution has become one of the common- 

 place matters of biological literature. The majority of the 

 synthetic works are concerned with the special problems 

 presented by what is after all a very extensive subject. The 

 object of this work is, like that of the majority of its prede- 

 cessors, a special one. It does not set out to review the problem 

 of variation in all its aspects, but to gather together all the 

 leading facts and principles that emerge from a study of varia- 

 tion and have any bearing on the causes of evolution. 



On account of the vast numbers of books and papers that 

 have been produced on evolution, some word of excuse is 

 perhaps needful in adding to the number. In spite of all 

 that has been written on this subject and the fresh prestige 

 which, after a period of intense criticism, the doctrine of 

 Natural Selection has acquired from mathematical and gene- 

 tical studies, we believe that the causes of evolution are still 

 obscure and the relative importance of the presumed causative 

 agencies is still to be assessed. We further believe that many 

 principles and much recorded data still need to be worked into 

 the general scheme of inquiry and that in a number of direc- 

 tions much more research is still necessary. Even such a sub- 

 ject as geographical distribution and variation, which might be 

 thought to be worn threadbare, is still in need of systematic 

 study. 



As we are mainly concerned in this work with the causes 

 of evolution it may well be asked whether a survey of this 

 subject based only on zoological data can be of much 

 assistance. We think that a comprehensive work including 

 both botanical and zoological data and principles of the kind 

 brought together here is eminently desirable. At the same 

 time we do not feel that such conclusions as we have formulated 



