G. J. EOMA.NES ON PHTSIOLOaiCAL SELECTION. 367 



exclusively to tlie reproductive system. Surely, therefore, what we 

 first of all require in a theory of the origin of species is an expla- 

 nation of this relatively constant or general distinction. But 

 this is just what all previous theories fail to supply. Natural 

 selection accounts for some among the many secondary distinc- 

 tions ; but is confessedly unable to account for the primary 

 distinction. The same remark applies to sexual selection, use 

 and disuse, economy of growth, correlated variability, and so 

 forth. Even the prevention of intercrossing by geographical 

 barriers is unable to explain the very general occurrence of some 

 degree of sterility between two allied varieties, which have 

 diverged sufficiently to take rank as different species. All these 

 theories, therefore, are here in the same predicament : they 

 profess to be theories of the origin of species, and yet none of 

 them is able to explain the one fact which more than any other 

 goes to constitute the distinction between species and species. 

 The consequence is that most evolutionists fall back upon a 

 great assumption : they say it must be the change of organiza- 

 tion which causes the sterility ; it must be the secondary distinc- 

 tions which determine the primary. But the contrary proposition 

 is surely at least as probable, namely, that it is the sterility which, 

 by preventing intercrossing with parent forms, has determined 

 the secondary distinctions ; or, rather, that this has been the 

 original condition to the operation of the modifying causes in all 

 cases where free intercrossing has not been otherwise prevented. 

 For, obviously, it is a pure assumption to say that the secondary 

 diiferences between species have been historically prior to the 

 primary difference, and that they stand to it in the relation of 

 cause to eifect. Moreover, the assumption does not stand the 

 test of examination, as I will now proceed to show. 



First, on merely a priori grounds, it scarcely seems probable 

 that whenever any * part of any organism is slightly changed in 

 any way by natural selection or any other cause, the reproductive 

 system should forthwith respond to that change by becoming 

 sterile with allied forms. What we find in nature is a more or 

 less constant association between the one primary distinction 

 and an endless profusion of secondary distinctions. Now, if this 

 association had been between the primary distinction and some 

 one, or even some few, secondary distinctions, constantly the 



* This appears to be what the theory req uires, seeing that all parts of organ- 

 isms are subject to secondary specific distinctions. 



