THE DISTRIBUTION OF VARIANTS IN NATURE 105 



heron x with the acute local differentiation of the sedentary 

 wren. Schmidt (191 8, p. 112) in the same way contrasts the 

 homogeneity of the Common Eel population with the acutely 

 diversified races of the localised Blenny (^parces), and Burck- 

 hardt (1900) shows that the cyclic and acyclic species of 

 Crustacea in Swiss lakes exhibit analogous differences in 

 the degree to which they form local races. 



It will at once strike the critic that, if Rensch's theory is 

 correct, the proneness or inability to form local or geographical 

 races must be the resultant of a number of conflicting tenden- 

 cies. Thus animals like land molluscs by their sedentary 

 habits should be especially prone to form local races, yet this 

 factor may be more than counterbalanced by a marked 

 liability to accidental transport arising from their small size 

 and mode of life. Small mammals, on the other hand, which 

 are more or less localised and have a limited range, are less 

 prone to be transported, so that they should form conspicuous 

 local races. Finally, large mammals and certain kinds of 

 birds, though they have a wider range, are obviously not 

 prone to wide accidental dispersal, so that they should 

 form larger, but still distinct (geographical) groups. Probably 

 Rensch would hold that the greater range of the last two 

 groups is set off by their localised breeding habits. It will 

 be noted that the contrast is not between, e.g., birds and 

 molluscs, but between widely ranging and sedentary forms 

 even of the same group. It will also be seen that wide-ranging 

 habits in the Mammalia should have the same effect as small 

 size in the Mollusca, viz. the restriction of local variation. 

 As the distinction between forms which vary geographically 

 and those which do not must be based on a resultant of the 

 kind just suggested, we would expect to find very considerable 

 differences in the degree in which local or geographical races 

 are formed, according as one or another of the conditioning 

 factors is paramount. We must also take into account a 

 tendency to which little attention has been given, viz. the 

 inherent tendency of a species to vary. We will now review 

 some of the salient facts from these points of view. 



1 The example chosen is perhaps not very fortunate. The Common Heron 

 has a remarkably wide range, is migratory and shows little or no regional varia- 

 tion. Nevertheless, it is a bird of otherwise sedentary habits and evidently 

 conservative in its breeding habits, as many of the English heronries date back to 

 an ' immemorial antiquity ' (Nicholson, 1929, p. 270). 



