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? Or in what other way could it have
arisen, since scales are also passively useful parts? It is true that if
we are only to call adaptation what has been acquired by the species we
happen to be considering, there would remain a great deal that could not be
referred to selection; but we are postulating an evolution which has
stretched back through aeons, and in the course of which innumerable