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.) 
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 
tudiment 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 Watzre, Jan. 14th, 1892.) 
