ME. G. J. ROMANES ON PHTSIOLO&ICAL SELECTION. 359 



incompatibility should ever be exclusively individual than that it 

 should be racial ; and therefore, as likewise remarked, I regard 

 these cases of individual incompatibility as of value to my theory 

 chiefly because they prove the actual occurrence of the variation 

 which the theory requires, and this as suddenly or spontaneously 

 arising in the highest degree of efficiency. But I will now 

 adduce evidence to show that a state of matters more or less 

 similar may be proved to obtain throughout a whole breed or 

 strain, so that we then have, not merely individual incompati- 

 bility, but what may be termed racial incompatibility ; and 

 therefore that we are on the highroad to the branching-place of 

 a new species. Here I will again quote my facts from Darwin, 

 partly because he has so profoundly studied the subject of 

 variation, but chiefly because, wherever it is possible, I desire to 

 rely upon his authority. 



In the ninth chapter of the ' Origin of Species,' and in the 

 nineteenth chapter of the ' Variation of Plants and Animals 

 under Domestication,' Mr. Darwin adduces miscellaneous evidence 

 of the fact that in many cases varieties of the same species exhibit a 

 higher degree of fertility within themselves than they do with one 

 another. In this respect, therefore, they resemble natural species. 

 Inasmuch, however, as they are not natural species, but domesti- 

 cated varieties (or the changed descendants of one natural species), 

 they are here available as evidence to prove what I have just called 

 racial incompatibility, due to the change which has been efiected in 

 their reproductive systems. It makes no difference whether we 

 regard this change as due to intrinsic or to extrinsic causes ; in 

 either case the racial incompatibility is the same, and this is all 

 that the theory of physiological selection requires. Take, for ex- 

 ample, the following case which, as Mr. Darwin says, "is the result 

 of an astonishing number of experiments made during many years 

 on nine species of Verbascum, by so good an observer and so 

 hostile * a witness as GTartner : namely, that the yellow and 

 white varieties when crossed produce less seed than the similarly 

 coloured varieties of the same species ; " and elsewhere he quotes 

 a statement from the same authority to the effect that the blue 

 and red varieties of the pimpernel are absolutely sterile together, 

 while each is perfectly fertile within itself. So that in these 

 cases we have a marked degree of racial incompatibility between 



* " Hostile " because G-artner believed that the distinction between species 

 and vai'ieties in respect of sterility is more absolute than Darwin believed. 



