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, the flat barbs, their short, hooked barbules
which, in the flight-feathers, hook into one another with just
sufficient firmness to resist the pressure of the air at each wing-beat,
the lightness and firmness of the whole apparatus, the elasticity of
the vane, and so on. And yet all this belongs to an organ which is only
passively functional, and therefore can have nothing to do with the
LAMARCKIAN PRINCIPLE. Nor can the feather have arisen through some
magical effect of temperature, moisture, electricity, or specific
nutrition, and thus selection is again our only anchor of safety.

But--it will be objected--the substance of which the feather consists,
this peculiar kind of horny substance, did not first arise through
selection in the course of the evolution of the birds, for it formed the
covering of the scales of their reptilian ancestors. It is quite true
that a similar substance covered the scales of the Reptiles, but why
should it not have arisen among them through selection?