382 ME. a. J. ROMANES ON PHTSIOLOaiCAL SELECTIOBf. 



when once this sterility is given as an independent variation, 

 the forms — though not necessarily the individuals — which profit 

 by it would be favoured by natural selection in their competition 

 with other forms which do not present such variation. In short, 

 once let intercrossing with the parent-type be prevented by phy- 

 siological selection, and the field is at once thrown open to the 

 further or cooperating influence of natural selection — whether 

 this be eifected directly, as here supposed, or indirectly by modi- 

 fying the reproductive system through the rest of the organism, 

 as previously supposed. Later on, under Divergence of Character, 

 I will show another and much more important respect in which 

 physiological selection, by preventing intercrossing with parent 

 forms, is able to assist natural selection in the differentiation of 

 specific types. 



Aegitment peom the Inxjtilitt of Specific 



DlFEEEENCES. 



With reference to inutility, after what has already been said, 

 I will only repeat this somewhat important question, — "Why is it 

 that apparently useless structures and instincts occur in such 

 profusion among species, in much less profusion among genera, 

 and scarcely at all among families, orders, and classes ? To this 

 question the Darwinist can only answer that the utility of ap- 

 parently useless structures really is less than that of structures 

 whose utility is observable. For although the argument from 

 ignorance may be available up to a certain point, it clearly 

 cannot be available to the extent of showing why useful struc- 

 tures within the Hmits of species should have their utility more 

 disguised than useful structures elsewhere. Hence the Darwinist 

 can only conclude that, at all events the majority of structures 

 which he assumes to be useful in the case of species are not seen 

 by him to be useful, because their utility actually is less than in 

 the case of structures distinctive of genera, families, and so forth. 

 He must argue that the points wherein species differ from species 

 — being points of smaller detail than those which serve to dis- 

 tinguish genera, families, &c. — present less opportunity of use- 

 fulness ; and, therefore, as a rule, actually are of too little use 

 to admit of their utility being diagnosed, although not of so 

 little use as to have prevented their development by natural 

 selection, which is a better diagnostitian of utility than the 

 naturalist. But how much more probable is the answer which 



