caused by change of function. So is it with animals to a large
extent, if not to the same extent. Though we have proof that by rough
usage the dermal layer may be so excited as to produce a greatly
thickened epidermal layer, sometimes quite horny; and though it is a
feasible hypothesis that an effect of this kind persistently produced
may be inherited; yet no such cause can explain the carapace of the
turtle, the armour of the armadillo, or the imbricated covering of the
manis. The skins of these animals are no more exposed to habitual hard
usage than are those of animals covered by hair. The strange
excrescences which distinguish the heads of the hornbills, cannot
possibly have arisen from any reaction against the action of surrounding
forces; for even were they clearly protective, there is no reason to
suppose that the heads of these birds need protection more than the
heads of other birds. If, led by the evidence that in animals the amount
of covering is in some cases affected by the degree of exposure, it were
admitted as imaginable that the development of feathers from preceding
dermal growths had resulted from that extra nutrition caused by extra
superficial circulation, we should still be without explanation of the
structure of a feather. Nor should we have any clue to the specialities
of feathers--the crests of various birds, the tails sometimes so
enormous, the curiously placed plumes of the bird of paradise, &c., &c.
Still more obviously impossible is it to explain as due to use or disuse
the colours of animals. No direct adaptation to function could have
produced the blue protuberances on a mandril's face, or the striped hide
of a tiger, or the gorgeous plumage of a kingfisher, or the eyes in a
peacock's tail, or the multitudinous patterns of insects' wings. One
single case, that of a deer's horns, might alone have sufficed to show
how insufficient was the assigned cause. During their growth, a deer's
horns are not used at all; and when, having been cleared of the dead
skin and dried-up blood-vessels covering them, they are ready for use,
they are nerveless and non-vascular, and hence are incapable of
undergoing any changes of structure consequent on changes of function.

Of these few then, who rejected the belief described by Professor
Huxley, and who, espousing the belief in a continuous evolution, had to
account for this evolution, it must be said that though the cause
assigned was a true cause, yet, even