342 ME. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 



that smaller changes of organic type, when produced by natural 

 selection and now known as species, should so generally be 

 attended with this result ? Or, as Mr. Darwin himself expresses 

 it, " the real difficulty in our present subject is not, as it appears 

 to me, why domestic varieties have not become mutually infertile 

 when crossed, but why this has so generally occurred with 

 natural varieties, as soon as they have been permanently modified 

 in a sufficient degree to take rank as species." 



Here, then, we have the core of the problem ; and it is just 

 here that Mr. Darwin's explanations fail. For he candidly says, 

 " We are far from precisely knowing the cause ; " and the only 

 suggestion he adduces to account for the fact is, that varieties 

 occurring under nature " will have been exposed during long 

 periods of time to more uniform conditions than have domesticated 

 varieties ; and this may well make a wide difference in the 

 result." I need scarcely wait to show the feebleness of this 

 suggestion. When we remember the incalculable number of 

 animal and vegetable species, living and extinct, we immediately 

 feel the necessity for some much more general explanation of 

 their existence than is furnished by supposing that their mutual 

 sterility, which constitutes their most general or constant distinc- 

 tion, was in every case due to some incidental effect produced on 

 the generative system by uniform conditions of life. To say 

 nothing of the antecedent improbability, that in all these millions 

 and millions of cases the reproductive system should happen to 

 have been affected in this peculiar way by the merely negative 

 condition of uniformity, there is, as it. seems to me, the over- 

 whelming consideration that, at the time when a variety is first 

 forming, this condition of prolonged exposure to uniform condi- 

 tions of life must necessarily be absent as regards that variety ; 

 yet this is just the time when we must suppose that the infertility 

 with its parent form arose. For, if not, the incipient variety 

 would at once have been re-absorbed into the parent form by 

 intercrossing, as we shall see more fully under the next head of 

 this criticism. 



In view of these considerations I conclude, that while Mr. 

 Darwin has given the best of reasons to show why domesti- 

 cated varieties have so rarely become sterile inter se, he has 

 entirely failed to suggest any reason why this should so generally 

 have been the case with natural species. 



