Morphology. 51 
the whole outline of the animal more fish-like in 
shape. This is the stage which we actually observe 
in the seals, where the hind legs, although retaining 
all their typical bones, have become shortened up 
almost to rudiments, and directed backwards, so as 
to be of no use for walking, while serving to complete 
the fish-like taper of the body. (Fig. 2.) But in the 
whales the modification has gone further than this, 
so that the hind legs have ceased to be apparent 
externally, and are only represented internally—and 
even this only in some species—by remnants so 
rudimentary that it is difficult to make out with 
certainty the homologies of the bones; moreover, 
the head and the whole body have become com- 
pletely fish-like in shape. (Fig. 3.) But profound 
as are these alterations, they affect only those parts 
of the organism which it was for the benefit of the 
organism to have altered, so that it might be adapted 
to an aquatic mode of existence. Thus the arm, 
which is used as a fin, still retains the bones of the 
shoulder, fore-arm, wrist, and fingers, although they 
are all enclosed in a fin-shaped sack, so as to render 
them useless for any purpose other than swimming. 
(Fig. 4.) Similarly, the head, although it so closely 
resembles the head of a fish in shape, still retains 
the bones of the mammalian skull in their proper 
anatomical relations to one another; but modified 
in form so as to offer the least possible resistance 
to the water. In short, it may be said that all the 
modifications have been effected with the least pos- 
sible divergence from the typical mammalian type, 
which is compatible with securing so perfect an 
adaptation to a purely aquatic mode of life. 
E 2 
