78 THE VARIATION OF ANIMALS IN NATURE 



of the so-called ' Laws ' of Allen, Bergmann and Gloger. In 

 some instances these ' trends ' are not obviously correlated 

 with environmental gradients (Swarth, I.e. pp. 98-100 ; 

 Hewitt, 1925, p. 263; Snodgrass, 1903, p. 411). The two 

 last-named writers attribute the series (in scorpions and birds) 

 to successive waves of migration. Hewitt (I.e. p. 274) speci- 

 fically states that the series he studied are phylogenetic. 

 Hutchinson (1929, p. 444) records an interesting trend from 

 west to east in South Africa among the Notonectidae, in which 

 three subspecies of Micronecta piccanin form a series, though 

 the typical form M. piccanin piccanin is found unmodified along 

 the whole trend. Swarth (I.e. p. 92) notes that a trend may 

 be composed of successive areas of subspecific or racial stability 

 separated by narrow areas of intergradation. 



2. The very general occurrence of local and geographical 

 races is discussed later on (p. 104). It should, however, 

 be pointed out here that into the formation of some groups 

 more than one factor probably enters, viz. differentiated 

 environments (the effects of which may be inherited or not), 

 isolation, mode of reproduction and inheritance. How far 

 adaptation to local conditions enters into their formation is 

 considered in Chapter VII. 



3. It has been shown (Chapters I and II) that there 

 is a great lack of knowledge as to how far the variation of 

 animals in nature is heritable or not and whether the very 

 obvious plasticity of form and habit is of any moment in 

 evolution. It has also been noted that there is among taxono- 

 mists and other students a rough-and-ready acceptance of 

 the distinction between fluctuations and heritable variation, 

 though there is no criterion for deciding between them other 

 than the very small number of experiments and rather dubious 

 analogies (Chapter I). All generalisations based on the facts 

 of local and geographical variation labour under this initial 

 disadvantage. There have, it is true, been cited a number of in- 

 stances in which the heritable or non-heritable nature of variants 

 has been satisfactorily determined. But it is reasonable to 

 ask— what inferences are to be drawn from perhaps 20 or 30 

 experiments, when our generalisations should cover the whole 

 range of recorded variation ? If modern Biology elects to 

 stand by the criterion of experiment in what, after all, consti- 

 tutes one of its most important fields of evolutionary research, 



