ADAPTATION 357 



especially in mountainous country. Of course it could be 

 maintained that each part of the specific range, characterised 

 by certain limiting environmental conditions, was inhabited 

 by a specially adapted race of the species. But, though this 

 may be true to some extent, it is a very large assumption to 

 suggest that such racial specialisation is so general as to lead 

 to close adaptation in all parts of the range. In fact, where 

 extensive division into races has occurred, as in some rodents 

 or humble bees, it appears much more likely that geographical 

 isolation has been the important factor, and adaptation to 

 special local conditions, if it has occurred at all, is at any rate 

 unrecognisable. We may consider this problem in a particular 

 instance. Filipjev (1929) has shown in his study of the chief 

 insect pests of the U.S.S.R. that each main Russian life zone 

 may be distinguished not merely by certain endemic or typical 

 species, but by the pests which do most damage in them. In 

 fact, the latter, ' dynamical ' definition of the zones is more 

 satisfactory than the former, or ' static,' since very few species 

 are literally confined to one zone. The Noctuid moth Feltia 

 segetum, for instance, does serious damage in the West Siberian 

 Forest zone and in the Middle subzone of the Steppe ; in the 

 former, more northern region, it is single-brooded, in the latter 

 double-brooded. Its complete range covers a very much 

 larger area, including the districts lying between those where 

 damage is done. Presumably in the intervening country it is 

 single-brooded in bad years and double-brooded in good 

 ones : such facultative increase in brood number is very 

 common in Lepidoptera. Even in the areas where the damage 

 is serious the degree of severity of outbreaks depends on climatic 

 conditions (e.g. rainfall), which may be more or less propitious 

 in different years. Evidently there is some adaptation of the 

 moth to varying conditions, but its range is too large and the 

 climate throughout the latter too variable for the adaptation 

 to be very close, except in some years or in certain limited 

 districts. 



In the previous paragraph we have illustrated a well- 

 known phenomenon of geographical distribution, viz. that 

 species have areas of optimum conditions surrounded by 

 zones in which the environment becomes progressively more 

 unsuitable and the species rarer. This suggests an examination 

 of what is implied by ' optimum conditions.' The life of an 



