76 THE VARIATION OF ANIMALS IN NATURE 



and passing through the local race to the subspecies. 1 In this 

 system we see groups progressively diverging either in more 

 characters or in the amplification of individual differences. 

 So far the bulk of our knowledge of these processes is concerned 

 with structural divergence, but there is strong evidence for 

 the occurrence of ' races ' which differ from one another in 

 single features of habit, food-preference and physiological 

 activity. Still further divergence is seen in the groups usually 

 recognised as species which contain a number of distinct 

 but intergrading subordinate elements of the various kinds 

 described above. Species may be more or less homogeneous 

 or they may be markedly diversified by sharply cut constituent 

 elements (Rassenkreise). Palaeontological evidence suggests 

 that historically considered the various individual character- 

 sequences within a group do not develop at the same rate. 

 This principle can probably be harmonised with the results 

 of neontology by reference to the observed fact that different 

 elements (e.g. colonies) exhibit different proportions of the 

 same stock of variants and the theoretical assumption that 

 new mutations occur at different parts in and spread slowly 

 through a population. 



1 Sometimes a form is given subspecific rank because it covers a wide area, 

 although it differs from its nearest ally in very minute details. On the other 

 hand, a well-marked variety with a very restricted range might not be given the 

 same rank, chiefly because, on the whole, fewer workers will be interested in 

 a form found only in a small area. 



