1 32 THE VARIATION OF ANIMALS IN NATURE 



is some evidence that even the main features of the habits of 

 ants were established at this early date, though it appears that 

 the polymorphism of the workers was not developed till the 

 Pleistocene. Apparently the species and genera of ants were 

 established at a much earlier date than those of several other 

 groups. If such a species as Ponera coarctata (Wheeler, I.e. p. 174) 

 has really existed with little change from the Lower Oligocene, 

 then only the most permanent geographical barriers would 

 have any effect on its divergence. Unless permanent biological 

 isolation was set up, there would be ample time for two isolated 

 races to be joined together again in the course of so prolonged 

 a specific history. 



Lapouge (1902) has given some account of the beetles of 

 the genus Carabus found in the Mid-Pleistocene of Belgium. 

 In this genus the surface sculpture of the elytra is highly 

 distinct and provides some of the most important characters 

 for separating species and races. The fossil elytra could all 

 be referred to existing species, except in one case ; but the 

 sculpture was nearly always somewhat different, to an extent 

 which in a modern form would be considered deserving of a 

 varietal or racial name. 



Borodin (1927) has published some data on the Clupeid 

 fishes of the Caspian Sea and a neighbouring lake. Certain 

 subspecies have probably been isolated from one another since 

 the second interglacial period (ca. 350,000 years) . The changes 

 they have undergone are not yet very great. Analogous data 

 are recorded of another fish (Cottus) in the Swedish lakes 

 (Lonnberg, 1932) and of the prawn, Limnocalanus (Ekman, 



1913)- 



The British mammals provide perhaps the best material 



for an inquiry of this nature. The evidence for each species 



is given by Barrett-Hamilton and Hinton (1911-1921). Two 



main types of evidence are available. First, in numerous 



instances, an existing species is found fossil in the Pleistocene 



as an identical or a scarcely different form, and we have some 



idea as to the length of time the species has remained unaltered. 



Secondly, in a few specially valuable instances, a species which 



is now represented by a purely British race does not occur in 



the British Pleistocene, and must have evolved to the extent 



to which it differs from its continental representative since that 



period. 



