26 GERMINAL SELECTION. 



right variations at the right place. But just here, it 

 would seem, is the insurmountable barrier to the ex- 

 planatory power of our principle, for who, or what, 

 is to be our guarantee that dark scales shall appear at 

 the exact spots on the wing where the midrib of the 

 leaf must grow? And that later dark scales shall 

 appear at the exact spots to which the midrib must be 

 prolonged? And that still later such dark spots shall 

 appear at the places whence the lateral ribs start, and 

 that here also a definite acute angle shall be accurately 

 preserved, and the mutual distances of the lateral ribs 

 shall be alike and their courses parallel? And that 

 the prolongation of the median rib from the hind 

 wing to the fore wing shall be extended exactly to 

 that spot where the fore wing is not covered by the 

 hind wing in the attitude of repose? And so on. 



If I could go more minutely into this matter, I 

 should attempt to prove that the markings, as I have 

 just assumed, have not arisen suddenly, but were per- 

 fected very, very gradually ; that in one species they 

 began on the fore wing and in another on the hind 

 wing ; and that in many they never until recently pro- 

 ceeded beyond one wing, in other species they went 

 only a little way, and in only a few did they spread 

 over the entire surface of both wings. 



That these markings advanced slowly and gradually, 

 but with marvelous accuracy, is no mere conjecture. 

 But it follows that the right variations at the right 

 places must never have been wanting, or, as I ex- 

 pressed it before: the useful variations were always 

 present. But how is that possible in such long exten- 

 sive lines of dissimilar variations as have gradually 

 come to constitute markings of the complexity here 

 presented? Suppose that the useful colors had not 



