s required for terrestrial locomotion becoming better adapted for
aquatic locomotion, and 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 completely 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 possible divergence from the typical mammalian type, which is
compatible with securing so perfect an adaptation to a purely aquatic
mode of life.

[Illustration: FIG. 2.--Skeleton of Seal, 1/8 nat. size. Drawn from
nature (_R. Coll. Surg. Mus._).]

[Illustration: FIG. 3.--Skeleton of Greenland Whale, 1/100 nat.
size. The rudimentary bones of the pelvis are shown on a larger
scale in the upper drawing. (From Prof. Flower.)]

[Illustration: FIG. 4.--Paddle of Whale compared with Hand of Man.
Drawn from nature (_R. Coll. Surg. Mus._).]

Now I have chosen the case of the whale and porpoise group, because they
offer so extreme an example of profound modification of structure in
adaptation to changed conditions of life. But the same thing may be seen
in hundreds and hundreds of other cases. For inst