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 adaptations took place, which
had not merely ephemeral persistence in a genus, a family or a class,
but which was continued into whole Phyla of animals, with continual
fresh adaptations to the special conditions of each species, family,
or class, yet with persistence of the fundamental elements. Thus the
feather, once acquired, persisted in all birds, and the vertebral
column, once gained by adaptation in the lowest forms, has persisted
in all the Vertebrates from Amphioxus upwards, although with constant
readaptation to the conditions of each particular group. Thus
everything we can see in animals is adaptation, whether of to-day, or
of yesterday, or of ages long gone by; every kind of cell, whether
glandular, muscular, nervous, epidermic, or skeletal, is adapted to
absolutely definite and specific functions, and every organ which is
composed of these different kinds of cells contains them in the proper
proportions, and in the particular arrangement which best serves the
function of the organ; it is thus adapted to its function.

All parts of the organ