VARIATION IN SPACE AT PRESENT EPOCH 135 



philosophical research into the variation of land 

 Molluscs might be repeated for all the other groups 

 of existing animals. It is perfectly exact that, 

 notwithstanding the polymorphism of certain types, 

 the limits of the great species always remain easy to 

 define. 



We recalled previously the regional races of the 

 common hare, Lepus timidus, from the temperate 

 zones of the centre of Europe to the Sahara. These 

 various races are, indisputably, only climatic 

 variations of a single species ; these variations, in 

 fact, only affect the external characteristics of size 

 and fur, and leave intact the deeper and more 

 intimate characteristics, such as dentition. But, 

 towards the northern limits of its domain, the Lepus 

 timidus comes into contact with another hare of 

 a more northern habitat, the Lepus variabilis, 

 which, outwardly, is not very different from it, 

 having only rather shorter ears and grey and black 

 fur like that of the common hare, which becomes 

 entirely white in winter, thus adapting itself to the 

 colour of the snowy countries it inhabits. If, 

 however, one compares the dentition, we observe 

 that the first upper molar in these two hares has a 

 different shape. In the Lepus timidus it is rounded 

 on the inside, while in the Lepus variabilis it shows 

 a deep furrow between two ridges of enamel. In 

 addition, the arch of the palate in the northern 

 species is larger, and does not become narrower 

 behind, as is the case with the common hare. These 

 characteristics of a deeper order than that of the 

 fur will always enable the two species to be dis- 

 tinguished. Even in the countries of the north of 



