46 THE VARIATION OF ANIMALS IN NATURE 



The whole weight of Rensch's argument depends, of course, 

 on accumulating a large number of examples which it is not 

 desirable to reproduce in the present chapter. 



His first point is the extremely gradual changes shown by 

 geographical races arranged along a climatic trend, e.g. in 

 the five races of Parus atricapillus between North Siberia and 

 the Rhine district the mean wing-length changes regularly 

 from 66-5 to 60 -5 mm. The changes are quite regular and 

 even the two extremes vary enough to overlap. A number of 

 similar examples is quoted. He stresses the fact that geo- 

 graphical variants are normally distinguished by several 

 characters rather than by one major character. Next are set 

 out numerous instances of parallel geographical variation, 

 showing that in any one district related forms tend to be all 

 modified in one direction. Examples in the Vertebrata, 

 Crustacea and Mollusca are given. Allen's Law concerning the 

 relative lengths of projecting parts is illustrated by a number 

 of tables. It is seen to hold for the tail-length in a variety of 

 mammals (chiefly rodents), when Alpine or northern races 

 are compared with the representative race occurring in 

 warmer districts. Interesting tables (pp. 149-15 1) show the 

 same relation between wing- and body-length in North 

 American Picidae, Bubonidae, etc. In 80 per cent, of the 

 species the wings are longer in the southern races. On p. 152 

 he turns to Gloger's Law, and in a table on p. 155 he shows its 

 application to twenty-five races of nine species of European 

 tits and tree-creepers. It is naturally more difficult to grade 

 species accurately according to colour. 



Rensch has collected together a bigger body of information 

 of this sort than has ever been presented before. For more 

 detailed information his book and bibliography must be 

 consulted. 



Certain analogous or additional examples, not mentioned 

 or not fully treated by Rensch, may be added. Alpatov ( 1 925, 

 1929) has recorded some interesting investigations on the Honey 

 Bee [Apis mellifera) in Europe. The data are very extensive 

 and have been subjected to rigorous statistical treatment. 

 He has been able to show that southern forms are smaller on 

 the average and have longer tongues, wider wings, longer 

 legs and small wax glands. The number of hooks on the 

 hind wings is greater and the colour is yellower. The change 



