MR. &. J. E0MANE8 ON PHYSIOLOGICAL SELECTION. 353 



or, if we like, on closely contiguous areas, two varieties of the 

 same species, eacli perfectly fertile within its own limits, while 

 absolutely sterile with one another. That is to say, there has 

 arisen between these two varieties a barrier to intercrossing 

 which is quite as effectual as a thousand miles of ocean ; the only 

 difference is that the barrier, instead of being geographical, is 

 physiological. 



Now, from this illustration I hope it will be obvious that 

 wherever any variation in the highly variable reproductive 

 system occurs, tending to sterility with the parent form while 

 not impairing fertility with the varietal form — no matter whether 

 this is due, as here supposed, to a slight change in the season of 

 reproductive activity, or to any other cause — there the physio- 

 logical barrier in question must interpose, with the result of 

 dividing the species into two parts. And it will be further 

 evident that when such a division is effected, the same conditions 

 are furnished to the origination of new species as are furnished 

 to any part of a species when separated from the rest by geo- 

 graphical barriers. For now the two physiologically divided 

 sections of the species are free to develop independent histories 

 without mutual intercrossing. 



Or, to state this suggestion in another way. If the suggestion 

 is well founded, it enables us to regard a large proportion, if not 

 the majority, of natural species as so many expressions of varia- 

 tion in the reproductive systems of their ancestors. When 

 accidental variations of a non-useful kind occur in any of the 

 other systems or parts of an organism, they are, as a rule, imme- 

 diately extinguished by intercrossing. But whenever they 

 happen to arise in the reproductive system in the way here 

 suggested, they must inevitably tend to be preserved as new 

 natural varieties or incipient species. Once formed as such, the 

 new natural variety, even though living upon the same area as 

 its parent species, will begin an independent course of history ; 

 and, as in the now analogous case of isolated varieties, will tend 

 to increase its morphological distance from the parent form, 

 until it eventually becomes a true species. At least it appears 

 to me obvious that in so many cases as variations of the kind in 

 question have taken place, in so many cases must the conditions 

 have been supplied to the formation of new species. Later on 

 I will show in more detail how these conditions have been 

 utilized. 



LINN. JOUKN. — ZOOLOGY, TOL. XIX. 28 



