a number of these transformations--the fish-like form of the
body, the hairlessness of the skin, the transformation of the
fore-limbs to fins, the disappearance of the hind-limbs and the
development of a tail fin, the layer of blubber under the skin, which
affords the protection from cold necessary to a warm-blooded animal,
the disappearance of the ear-muscles and the auditory passages, the
displacement of the external nares to the forehead for the greater
security of the breathing-hole during the brief appearance at the
surface, and certain remarkable changes in the respiratory and
circulatory organs which enable the animal to remain for a long time
under water. I might have added many more, for the list of adaptations
in the whale to aquatic life is by no means exhausted; they are found
in the histological structure and in the minutest combinations in the
nervous system. For it is obvious that a tail-fin must be used in
quite a different way from a tail, which serves as a fly-brush in
hoofed animals, or as an aid to springing in the kangaroo or as a
climbing organ; it will require quite different reflex-mechanisms and
nerve combinations in the motor centres.

I used this example in order to show how unnecessary it is to assume a
special internal evolutionary power for the phylogenesis of species,
for this whole order of whales is, so to speak, _made up of
adaptations_; it deviates in many essential respects from the usual
mammalian type, and all the deviations are adaptations to aquatic
life. But if precisely the most essential features of the organisation
thus depend upon adaptation, what is left for a phyletic force to do,
since it is these essential features of the structure it would have to
determine? There are few people now who believe in a phyletic
evolutionary power, which is not made up of the forces known to
us--adaptation and heredity--but the conviction that _every_ part of
an organism depends upon adaptation has not yet gained a firm footing.
Nevertheless, I must continue to regard this conception as the correct
one, as I have long done.

I may be permitted one more example. The feather of a bird is a
marvellous structure, and no one will deny that as a whole it depends
upon adaptation. But what part of it _does not_ depend upon
adaptation? The hollow quill, the shaft with its hard, thin, light
cortex, and the spongy substance within it, its square section
compared with the round section of the quill,