32 THE GROWTH OF GROUPS 



if a species, defined in the conventional manner, is com- 

 pliant with the test or not, for we are never able to apply 

 it strictly, i.e. in respect to every individual of the 

 species. 



Not until we know completely the physiological basis 

 of fertility and sterility in the individual shall we know 

 the origin of species in a strict sense, nor will our know- 

 ledge of the subject be complete until we can render a 

 pair of fertile animals sterile with all others. But this 

 question is apart from that of the early origin of morpho- 

 logical groups which is the subject of this book. It seems 

 obvious that no group can remain distinct as a group, 

 for any length of time, unless the members of it are sterile 

 with all animals outside the group. In other words, the 

 unknown facts of sterility would, if known, account for 

 the permanence of groups, more than for their origin. 



Only permanent groups should be considered worthy 

 of a specific name. Discussions concerning the dis- 

 tribution of species are futile unless the term is restricted 

 to groups that are permanent through countless genera- 

 tions. It will be seen that this book scarcely deals with 

 the origin of species, using the term in this restricted 



THE TRUTH OF THE SELECTION THEORY 



All biologists are agreed that the selection theory is 

 broadly true ; that gaps have appeared between the 

 various forms of animal life because of the extinction 

 of the intermediate forms which filled those gaps. For 

 the moment, the origin of species may be regarded as the 

 origin of the gaps between them, just as the origin of 

 the hills in a range is the origin of the gaps between the 



