dification has gone even further than this, so that the
hind legs have ceased to be apparent externally, and are only
represented internally by remnants so rudimentary that it is impossible
to make out with certainty the homologies of the bones; moreover, the
head and the whole body have become completely fish-like in shape. But
profound as these changes are, they only affect 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 inclosed in a
fin-shaped sack, so as to render them quite useless for any other
purpose than swimming. 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 relation to one another, but
modified in form so as to offer the least possible amount of 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.

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 instance, to confine our
attention to the arm, not only is the limb modified in the whale for
swimming, but in another mammal--the bat--it is modified for flying, by
having the fingers enormously elongated and overspread with a membranous
web. In birds, again, the arm is modified for flight in a wholly
different way--the fingers here being very short and all run together,
and the chief expanse of the wing being composed of the shoulder and
fore-arm. In frogs and lizards, again, we find hands more like our own;
but in an extinct species of flying reptile the modification was
extreme, the wing having been formed by a prodigious elongation of the
fifth finger, and a membrane spread over it and the rest of the hand.
Lastly, in serpents the hand and arm have disappeared altogether.

Thus, even if we confine our attention to a single structure, how
wonderful are the modifications which it is seen to undergo, although
never losi