THE DISTRIBUTION OF VARIANTS IN NATURE 123 



mosquitoes are similarly dimorphic. The common British 

 moths Acronyctapsi and A. tridens may also be mentioned. The 

 larvae differ sharply in colour, though the adults arc separable 

 only by the genitalia. In all such cases it is logical to claim 

 that evolution has progressed further in the larvae than in the 

 adults, just as in biological races evolution has been in the 

 direction of physiological rather than structural divergence. 

 It is of some interest to show that the tendency to form local 

 populations does not affect only structural characters. The 

 existence of biological races evidently provided partial proof 

 of this, but we may add a number of other instances of local 

 segregation of what may be called ' non-taxonomic ' characters. 

 Local variation in the extent of sexual dimorphism is not 

 at all rare, but is best considered a special case of normal 

 group-formation in structural characters. There is much 

 variation in seasonal occurrence in most insects with a wide 

 range. It is usually unknown to what extent this character 

 is due to the direct action of the environment. Probably the 

 genetic element is larger than is commonly supposed. While 

 often the number of broods gradually increases as one goes 

 south, in other cases closely allied forms have a different life- 

 cycle in the same district. Sometimes the effect of temperature 

 is reversed. Thus, in gall-wasps, Kinsey (1930) finds that 

 the species emerge earlier in the north, and Willey (1930, 

 pp. 79-80) records a comparable condition in Copepods, in 

 which growth is faster in the north. Many butterflies which 

 have more than one brood a year show marked differences 

 between the spring and summer broods. Such seasonal 

 change is much subject to local variation and may be almost 

 absent in some parts of the range (cf. Ford, 1924). 



Gurney (1929) shows that some Copepods are locally 

 dimorphic in size, while elsewhere this character is distributed 

 in a normal curve. In some species one sex alone shows the 

 dimorphism. This may be compared to the dimorphism in 

 the males of the Common Earwig (Forficula auricularia) . Bateson 

 and Brindley (1892) showed that in some localities high males 

 were much more prevalent than in others. Stephenson (1929) 

 records various methods of reproduction separating species of 

 Sagartia. Amongst eight species there are five methods. 



Local variations in the sex-ratio are also well known. 

 The subject has been dealt with at some length by Yandel 



