1890 PHYSIOLOGICAL SELECTION 183 



phatically is not my theory. Physiological isola- 

 tion I regard as having morphological consequences 

 precisely analogous to those of geographical isolation ; 

 and you would not think of arguing that there must 

 be a separate geographical isolation for every slight 

 change of structure — for example, that a peculiar 

 species of plant growing on a mountain top must 

 have had one isolation to explain its change of 

 form, and another isolation to explain its change of 

 colour. 



Lastly, if you will look up Hilgendorf's paper 

 about these snails of Steinheim, I think you will find 

 it impossible to suppose that all these little changes 

 (thus arising at long intervals in crops) can have 

 been useful. Or, if you can still doubt, look up the 

 closely analogous but much larger case of the ammo- 

 nites investigated by Neumayr and Wurtenberger. 



What I meant about the sexual system being 

 specially liable to variation is, that it is specially 

 liable to variation in the way of sterility. In other 

 words, changed conditions of life more readily effect 

 variations in the primary functions of the sexual 

 system than they do in general morphology. But at 

 the same time, I quite agree with your view that in 

 the last resort all changes of structure may be 

 regarded as due to variations of this system. And, 

 as you will see by turning to pp. 371-72 of my paper, 

 important capital is made out of this doctrine. 



Now about making too much of the inutility of 

 specific characters ; if I do so, it is erring on the 

 side of natural selection ; for it clearly follows from 

 this theory that, if there are any useless struc- 



