The Tests of Evolution 243 



idea of great and sudden transformations that Darwin 

 wrote: "To admit all this is, as it seems to me, to enter 

 into the realms of miracle and to leave those of sci- 

 ence." The mutation theory is in favour with the genet- 

 icists, who have found that definite variations occur 

 and are definitely inherited. But the geneticists are 

 puzzled to suggest how these variations could become 

 specific characters, common to all the members of a 

 species, seeing that they are not adaptive, and there- 

 fore could not be selected. 



Systematists attach little importance to interspecific 

 steriHty; they know that Darwin showed that between 

 alHed species there are all gradations, from complete 

 sterility to complete fertility. But for the geneticists 

 sterility is all-important — it is their one hope of pro- 

 ducing the semblance of a species — and they proclaim 

 that the event for which they are waiting is the pro- 

 duction of a variety which is sterile with the parent 

 form. That great event, if and when it occurs, will 

 leave me cold; in my opinion, it will have about as 

 much relation to the orgin of species as the occurrence 

 of albinos has to the coloration of arctic animals — that 

 is to say, no relation whatever! 



My own work on the structure, classification, and 

 geographical distribution of fishes has led me to cer- 

 tain conclusions. I believe that the first step in the 

 origin of a new species is not a change of structure, 

 but the formation of a community, either through lo- 

 calization, geographical isolation, or habitudinal segre- 

 gation. I also think that specific characters may be 

 grouped as follows: they are either {a) useful, {b) 



