THE GROWTH OF GROUPS 135 



rubicunda occupying an area of seven or eight square miles 

 at Toluca. Can we escape from the conclusion that that 

 group arose from mutants ? Having seen the method of 

 origin of a single rubicunda, why is it necessary to assume 

 that there is another method ? 



Beetles of the kind rubicunda come into the world in 

 one of two ways, ordinarily from an egg laid by a beetle 

 of the kind rubicunda or extraordinarily from the egg of 

 one of the kind multitceniata. This we know, and we had 

 better hold fast to this knowledge and assume that these 

 are the only methods of arrival, until some one can 

 describe to us a third method in terms that we can under- 

 stand. It may be said that we do not understand the 

 arrival of rubicunda from multitceniata, but do we under- 

 stand the common event rubicunda from rubicunda ? We 

 do not understand either of them, but we know that they 

 occur, and that is what we require from a scientific point 

 of view. 



The facts relating to these two kinds of beetles will 

 suggest to any one that species arise from mutants, 

 though we know they may not convince him. But 

 supposing that conclusion is objectionable, where lies the 

 way of escape from it ? The readiest way is to argue that 

 the group of rubicunda found at Toluca was not really 

 a species ; that is to say, not sufficiently established to 

 be a real species. By thus altering our conception of 

 " species " we may subdue the thought that a species 

 had arisen from a mutant. Before we can decide the 

 question in a scientific manner we must therefore lay down 

 a numerical limit for species in general. If we say that a 

 group is not a species until it contains a million individuals, 

 we shall then be able to know in any case when a species 



