THE RABBIT. 131 



generic name, the other its own private specific name. 

 Thus the rabbit is called Lepus cuniculus, while the hare 

 is Lepus timidity. 



What is the exact amount of difference necessary to 

 constitute a difference of species ? To this question no 

 answer can be given. Every case must be decided on its 

 merits by the judgment of the biologist who is studying it, 

 and there are many known cases where one investigator 

 has made two or more species out of a set of animals which 

 another has put in one species. There are two theories as 

 to the origin of species, but neither affords an absolute 

 criterion. According to the first theory, every species is 

 descended from a pair of ancestors originally created out of 

 not-living matter : this theory draws a perfectly sharp line 

 between species and species, but unfortunately it makes the 

 evidence for specific distinctness quite inaccessible, since we 

 should never know that rabbit and hare were distinct 

 species unless the creation of their first ancestors had been 

 witnessed and described by a competent scientific investi- 

 gator, and the pedigree carefully recorded down to the 

 present day. 



The second theory regards species as gradually differen- 

 tiated by modification during successive generations, through 

 natural selection (and perhaps other processes). We have 

 seen that the rabbit-species is inevitably becoming 

 more and more fully adapted to its mode of life ; and 

 the same might be shown of the hare. Hence every suc- 

 cessive generation must increase the difference of average 

 structure between hare and rabbit. Conversely, if we go 

 back in time, every generation will show less difference 

 between the two species, until it is conceivable that we 

 might arrive at a common ancestral species adapted for a 

 life intermediate in character between that of the modern 

 rabbit and hare. Evidently, then, it is only the dis- 

 appearance of intermediate forms that makes rabbit and 

 hare appear as distinct as they are, so that an absolute 

 criterion again fails us. 



Whichever theory of the crigin of species we adopt, then, 

 we have to decide our species on practical grounds of con- 

 venience, For two sets of animals to be accounted distinct 



