1890 PHYSIOLOGICAL SELECTION 175 



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 steriKty. 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 inutihty 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- 

 tures at all, they ought to occur with (greater ?) 

 frequency among species, where (as ?) yet natural 

 selection has not had time to remove them. But I 

 cannot think I have here unduly favoured natural 

 selection. For although there are not a few instances 

 of apparently useless structures running through even 

 an entire class (as the ' Origin ' remarks), these are 

 not only infinitely less numerous than apparently 

 useless structures in species, but are also very much 

 more rarely trivial. 



Now the latter fact, coupled with that of the 

 greatly wider range of their occurrence, appears to 

 me intensely to strengthen ' the argument from 

 ignorance,' i.e. to give us much more justification for 

 beheving that they are now, or once were, of use. 



