60 Darwin, and after Darwin. 



to appear in any part of another, surely we should 

 expect that within the limits of the same type the 

 same typical structures should always be present. 

 Thus, remember what efforts, so to speak, have been 

 made to maintain the uniformity of type in the case of 

 the fore-limb as previously explained, and should we 

 not expect that in other and similar cases a similar 

 method should have been followed ? Yet we repeatedly 

 find that this is not the case. Even in the whale, as we 

 have seen, the hind-limbs are either altogether absent or 

 dwindled almost to nothing ; and it is impossible to 

 see in what respect the hind-limbs are of any less ideal 

 value than the fore-limbs which are carefully pre- 

 served in all vertebrated animals except the snakes, 

 and the extinct Dinornis, where again we meet in 

 this particular with a sudden and sublime indiffer- 

 ence to the maintenance of a typical structure. (Fig. 6.) 1 

 Now I say that if the theory of ideal types is true, we 

 have in these facts evidence of a most unreasonable in- 

 consistency. But the theory of descent with continued 

 adaptive modification fully explains all the known 

 cases; for in every case the degree of divergence from 

 the typical structure which an organism presents 

 corresponds, in a general way, with the length of time 

 during which the divergence has been going on. 

 Thus we scarcely ever meet with any great departure 

 from the typical form with respect to one of the 

 organs, without some of the other organs being so far 

 modified as of themselves to indicate, on the sup- 



1 It is, however, probable that all species of the genus retained a tiny 

 rudiment of wings in greatly dwindled scapulo-coracoid bones. And 

 Mr. H. O. Forbes has detected, in a recently exhumed specimen of the 

 latter, an indication of the glenoid cavity, for the articulation of an 

 extremely aborted humerus. (See Nature, Jan. *4th, 1892.) 



