THE DISTRIBUTION OF VARIANTS IN NATURE 1 1 1 



Certain other facts are also relevant. In the first place 

 the relatively small number of species in birds and mammals 

 has enabled much greater advance to be made in the study 

 of subspecific differences ; the definition of a large number of 

 geographical races does not of itself prove that this type of 

 variation is more common in these groups than in others in 

 which the number of species is very much greater. Again, 

 where the number of species is small, the systematist will tend 

 not to hesitate to introduce a new name for any apparently 

 stable local form. In such groups as the insects the species 

 are already so numerous that considerable evidence is needed 

 before definite named races will be published. In the Lepido- 

 ptera, in dealing with which authors have been less cautious, 

 considerable confusion has resulted. The local variation 

 is so great that it is a difficult and lengthy task to deal ade- 

 quately with even a single species, and, where species are 

 numerous, it is unlikely that more than a few have been 

 sufficiently studied for so-called ' races ' to be very clearly 

 defined. 



Secondly, a geographical race is commonly defined by the 

 average size, proportion or colour of certain parts. No one 

 (p. 69) supposes that geographical races are normally uni- 

 formly homozygous for merely a single differential factor, 

 so that the variation within the race cannot be regarded as 

 purely somatic. This implies that the race could be broken 

 up into a number of varieties differing slightly from one another 

 in the diagnostic race-characters. The average of these 

 varieties gives the race, because, being quantitative, these 

 characters can be given a mean value. But, in other cases, 

 as often in insects, a species consists of several rather sharply 

 discontinuous varieties. If these differ qualitatively, they 

 cannot be averaged : it is possible only to give the proportion 

 in which the different varieties occur in different parts of the 

 range. Difference in these proportions evidently defines a 

 race of exactly the same nature as described in the last para- 

 graph. But in normal taxonomic procedure the race described 

 there would receive a name, whereas in the second case each 

 of the distinct forms would receive a name, but there would 

 be no name for the various populations defined by consisting 

 of different proportions of the named forms. The example 

 of Harmonia axyridis ( (h), p. 103) exhibits this difficulty. 



