THE GROWTH OF GROUPS 31 



inbred stock can be revived by the admission of new 

 blood ? There is little certain evidence on this question 

 to help us. On the other hand, certain evidence is 

 afforded by the Australian rabbits. Somehow or other 

 those animals overcame the ill effect of inbreeding, 

 supposing that they ever felt it. Their case is not 

 unique. Darwin relates of a litter of tame rabbits, born 

 on a ship, which were released on a certain island. The 

 descendants of that single litter became so numerous 

 as to cause the inhabitants to abandon the island. It 

 is also certain that a pair of rats are able to give rise to a 

 group of many million descendants. Is it correct to say 

 that the members of such a group would still be practising 

 inbreeding after the twentieth generation ? In the 

 ordinary sense, many of them would be distantly related 

 to one another. 



PHYSIOLOGICAL SPECIES 



All bisexual animals which are mutually fertile and 

 produce fertile offspring, taken together, constitute a 

 physiological species. At present we do not know why 

 one pair of animals is fertile and another sterile. We 

 know, in a general way, that those which are nearly alike 

 are as a rule mutually fertile, while those which are unlike 

 are mutually sterile; but the likelihood of a pair being 

 fertile is not directly proportional to their resemblance, 

 in a strict sense. For most naturalists the test of a species 

 lies in the mutual fertility of its members and their 

 sterility with all other animals. A group which com- 

 plies with this test is in the usual sense a real species. 

 It is well known, however, that such groups are composite 

 as a rule in respect to their characters. We never know 



