64 The Selection Theory 
under water. I might have added many more, for the list of adapta- 
tions in the whale to aquatic life is by no means exhausted; they 
are found in the histological structure and in the minutest combina- 
tions 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 asa 
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 com- 
pared 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 
