362 MK. G. J. ROMAKES ON PHYSIOLOGICAL SELECTIOIT. 



This way of looking at tte matter may perhaps be rendered 

 more effective if we glance for a moment at the extraordinary 

 differences in the degrees of sterility which are manifested by 

 variations that have gone far enough to be ranked by naturalists 

 as undoubted species. For in this way we can see how impos- 

 sible it is to lay down any hard and fast distinctions between 

 species and varieties in respect of sterility, even though it has 

 always been the aim of naturalists to give primary importance to 

 this point. Now this difficulty is just what we ought to find, 

 according to my theory, as a very few words will be enough to 

 show. For, even if allied forms were always closely contiguous 

 forms, we should expect on this theory that great differences in 

 the degrees of sterility should be manifested by different species. 

 According to this theory, species are but records of a sufficient 

 degree of sterility having arisen with parent forms to admit of 

 the varietal form not becoming swamped by intercrossing. Now, 

 the degree of sterility required for this purpose would not be the 

 same in all cases, seeing that in some cases other conditions 

 might be present to assist in the prevention of intercrossing, as 

 we shall see later on. Moreover, in other cases the initial (or 

 the subsequently induced) degree of sterility may have been 

 greater than was required to effect the physiological separation 

 that took place. Lastly, when to these considerations we add 

 that allied species are not always necessarily contiguous species, 

 and therefore need never have had any opportunity of intercros- 

 sing (having originated independently from the same parent 

 form in different localities) — when we consider all these things, 

 we should expect to find the degrees of sexual incompatibility 

 between species highly variable. Or, in other words, we should 

 expect to find that the extreme varieties called species should 

 not exhibit an equal degree of incompatibility in all cases. And 

 this is just what we do find; or, as Mr. Darwin puts it, "the 

 sterility of various species when crossed is so different in degree, 

 and graduates away so insensibly, and, on the other hand, the 

 fertility of pure species is so easily affected by various circum- 

 stances, that for all practical purposes it is most difficult to say 

 where perfect fertility ends and sterility begins." 



But not only so. Among all the varieties in nature which are 

 extreme enough to be ranked as species, we might expect, upon 

 the theory of physiological selection, that some should have 

 developed sterility towards certain of their allies, while develop- 



