124 THE VARIATION OF ANIMALS IN NATURE 



(1928), who finds that in many Hymenoptera there is a 

 tendency for the species to be parthenogenetic in the northern 

 part of their range, but to reproduce normally in the south 

 (cf. also Brues, 1928). Poulton (1931) described similar local 

 anomalies in the sex-ratio in the Fijian butterfly, Hypolimnas 

 bolima. There appear to be a good number of instances of 

 insects which possess two types of females, male-producers 

 and female-producers, but the two types are not often geo- 

 graphically segregated. 



Summary 



Any account of variation is unfortunately limited by the 

 inability to present more than a small selection from the vast 

 mass of available data. It has been usual in the past (and 

 the practice is difficult to avoid) to construct all-embracing 

 theories on the basis of selected species or genera which supply 

 favourable data ; the theories based on the genetics of Droso- 

 phila or of Oenothera are cases in point. Obviously the best 

 method would be to treat all doubtful points statistically and 

 to state definitely that a particular type of variation occurred 

 in such-and-such a percentage. In the present state of taxo- 

 nomy no numerical statement of this sort is possible except 

 perhaps for a few well-worked groups. For, in the absence 

 of experimental investigations, it is often quite uncertain 

 whether particular variations are inherited, and moreover 

 the diverse types of variation encountered are very numerous 

 and difficult to classify, so that statistical treatment might in 

 any case be liable to serious errors. In the preceding account 

 we have tried to choose our examples fairly and not to pick 

 out merely those which support views we already hold on 

 other grounds. 



Up to the present we have not considered the effects of 

 isolation and the different ways in which it can be brought 

 about. Evidently isolation of one sort or another is a prime 

 factor in the process of group formation. Geographical 

 isolation is the type most easily recognised, and it is on this 

 account that taxonomists have evolved the conception of the 

 ' geographical race,' a term applied to minor categories, 

 whose ability to interbreed with their closest allies is held in 

 check only by more or less marked spatial separation. Other 



