INTRODUCTION 35 



in this way is due to the fact that the material to select 

 from is inexhaustible, and that immense periods of 

 time are available for the process of transformation. 



We see, therefore, that the changes that are brought 

 about by natural selection, through the emphasising of 

 insignificant characteristics, may become very striking. 

 The external difference between grey and white hares 

 is very considerable. It is true that one might claim 

 the white hares to be a variety of the grey ones, and 

 say that the difference between the two forms is not as 

 great as between the hare and the next species, the 

 rabbit. But is there any difference between a " variety " 

 and a "species"? Many writers understand by 

 "varieties" animals with somewhat different features 

 from those of the mother-species, but say that these 

 characteristics vacillate, and are not preserved in the 

 course of generations are not constant, as the phrase 

 is. We see that this test of the variety does not hold 

 good in the case of our white hares ; the whiteness is 

 constant, because all reversions are destroyed. And 

 when it is said that the variety can always pair with 

 the mother-species and produce fruitful offspring, which 

 cannot take place between two different species, the 

 statement is not entirely correct. Many species can be 

 coupled, and beget young that are capable of repro- 

 ducing. This is the case, for instance, with the wolf 

 and the dog, the carp and the crucian, and other 

 animals, which everybody recognises as distinct species. 



We can safely affirm to-day that there are no rigid 

 tests for determining whether certain types of animals 

 are species or only varieties ; in other words, there 



