ISOLATION 133 



The following data for British insectivores and rodents are 

 derived from Barrett-Hamilton and Hinton (1911-1921). 



(1) Adequate fossil data not available : 9 species. 



(2) Species not known in the Pleistocene, but now repre- 



sented by a distinct British race : 3 species (Common 

 Hare, Field-mouse (Microtus hirtus), and Water-rat 

 (with two races) ). 



(3) Form apparently identical with the modern repre- 



sentative known from at least Late Pleistocene : 

 (a) No British race : 4 species {Epimys rattus, Shrew, 

 Pigmy Shrew, Rabbit), (b) With a British race : 

 3 species (Irish Hare, Northern Field-mouse (Microtus 

 agrestis) , Apodemus flavicollis) . 



(4) Late Pleistocene form racially distinct : (a) No British 



race: 2 species (Mole, ? Water-shrew), (b) One 

 or more British races : 4 species (Apodemus sylvaticus 

 (2 races), Skomer Vole (3), Bank Vole, Orkney 

 Vole (5) ). 



The examples under (2) are particularly instructive, since 

 it is almost certain that fossils would have been found had the 

 animals been present in the Late Pleistocene. On the other 

 hand, since there is now a distinct British race, or, in the 

 Water-rat, two races, we can say that this degree of evolution 

 has taken place since the Pleistocene. 1 



In the six species included in (4) evolution has been rapid 

 enough to produce new races since the Late Pleistocene, while 

 in the seven species under (3) there has probably not been 

 much change since the Pleistocene. 



Evidently the data are not sufficient to support much 

 speculation, but they do at least suggest that in the rodents 

 and insectivores, of which at least the former group appears 

 to evolve very rapidly, the evolution of a new race normally 

 takes an interval of time not much shorter than that intervening 

 between the end of the Pleistocene and the present day. This 

 period of time is well known to have been sufficient for con- 

 siderable changes in geographical barriers and we may surmise 

 that, with evolution working at this rate, intrinsic methods 



1 An alternative hypothesis would be that the British form had remained 

 unaltered, and that it was the continental representatives that had changed. 



K 2 



