88 Darwin, and after Darwin. 



rence present constant varieties ; (3) these varieties, 

 nevertheless, may be morphologically so sh'ght as to 

 be almost imperceptible ; (4) they occupy common 

 areas and grow in intimate association ; (5) although 

 many of them have undergone so small an amount 

 of morphological change, they have undergone a sur- 

 prising amount of physiological change ; for (6) not 

 only do very many of these varieties come true to 

 seed ; but, (7) when they do, they are always more or 

 less cross-infertile inter se. 



Now, it is self-evident tliat everyone of these seven 

 points is exactly what the theory of physiological 

 selection requires, while there is not one of them 

 which it does not require. For if the theory be 

 sound, we should expect to find large numbers of 

 species belonging to numerous and varied orders 

 of plants presenting constant varieties on common 

 areas ; we should expect this to be a highly general, 

 though not a universal, rule ; and we should expect 

 it to apply only to species which are indigenous. More- 

 over, we should expect these varieties, although but 

 slightly differentiated morphologically, to present a 

 great differentiation physiologically — and this in the 

 special direction of selective fertility, combined, of 

 course, with heredity. 



On the other hand, as I have said, this catalogue 

 of evidences leaves nothing to be supplied. It gives 

 us all the facts — and no more than all the facts — 

 which my paper on Physiological Selection anticipated 

 as the eventual result of a prolonged experimental 

 research. And if I have to regret my ignorance of 

 these facts when that paper was published, at any 

 rate it now furnishes the best proof that my anticipa- 



