376 ME. a. J. ROMAisrES on physiological selection. 



principles we must look for our ultimate explanation of the origin 

 of species *. 



If we thus regard sterility between species as the result o£ what 

 I have called a local variation arising only in the reproductive 

 system, whether induced by changes taking place in other parts 

 of the organism, to changes in the conditions of life, or to changes 

 inherent in the reproductive system itself, we can understand 

 (a) why such sterility rarely, though sometimes, occurs in our 

 domesticated productions ; (5) why it so generally occurs in some 

 degree between species ; and (c) why as between species it occurs 

 in all degrees. 



(a) It rarely occurs in our domesticated productions, because 

 it has never been the object of breeders or horticulturists to pre- 

 serve this kind of variation. Yet it sometimes does occur in some 

 degree among our domesticated productions, because the changes 

 produced on other parts of the organism by artificial selection do, 

 in a small percentage of cases, react upon the reproductive system 

 in the way of tending to produce sterility with the parent form, 

 without lessening fertility with the varietal form. Again (6), this 

 particular condition of the reproductive system is so generally 

 characteristic of species, simply because, as a general rule, it is 

 owing to this condition that species exist as species ; any varia- 

 tion, therefore, towards this condition, howsoever produced, must 

 always have been preserved by physiological selection, with the 

 result of a new specific form to record the fact. And, lastly (c), 

 this particular variation in the reproductive system has taken 

 place under nature in such a number of degrees, from absolute 

 sterility between species up to complete, or even to more than 

 complete fertility, because natural species, while being records 

 of this particular kind of variation, are likewise the records of all 

 degrees of such variation which have proved sufficient to prevent 

 overwhelming intercrossing with parent forms. Sometimes this 

 degree has been less than others, because other conditions — cli- 

 matic, geographical, habitational, physiological, and even psycho- 

 logical — have co-operated to prevent intercrossing, or even to 



* In order to avoid needlessly confusing the foregoing argument, I have 

 omitted to notice that geographical barriers serve the same function as physio- 

 logical barriers ; and also that secondary distinctions caused by use and disuse 

 do not require to be protected from the levelling effects of intercrossing. But, 

 as will be seen from the next succeeding paragraphs, these considerations are in 

 no way opposed to my theory. 



