BURBANK PLUMS 349 



To be sure, the tendency to grow barriers be- 

 tween species is obvious enough, for everyone 

 knows that most of the others among our do- 

 mestic animals cannot interbreed at all. But, on 

 the other hand, if species are really only races 

 diverged from a common origin, as Darwin 

 thought, then there must have been a time 

 when those that are now widely separated 

 were nearer together and hence capable of 

 interbreeding. 



And as there are infinite gradations as to the 

 amount of the divergence between the extant 

 species of to-day, might we not reasonably sup- 

 pose there are many of these extant species that 

 have not yet diverged beyond the point of hybrid- 

 izing with the production of fertile offspring? 



TESTING THE THEORY 



Just how far I had been carried along such 

 lines of reasoning before undertaking to put the 

 matter to a test, it would perhaps be difficult or 

 quite impossible at this remote day to say with 

 certainty. 



But in any event my premonitions in the mat- 

 ter were sufficiently tangible to lead me, even 

 when scarcely more than a youth in Massachu- 

 setts, to attempt hybridizing experiments. And 

 the results of these experiments were sufficiently 



