Moreover in the deeper layers of the skin of adult birds, the muscular
network is, according to Leydig,[20] in a transitional condition;
the fibres exhibiting only indications of transverse striation.
Another explanation seems possible. We may admit that originally
the _arrectores pili_ were slightly acted on in a direct manner,
under the influence of rage and terror, by the disturbance
of the nervous system; as is undoubtedly the case with our
so-called _goose-skin_ before a fever-fit. Animals have been
repeatedly excited by rage and terror during many generations;
and consequently the direct effects of the disturbed nervous
system on the dermal appendages will almost certainly
have been increased through habit and through the tendency
of nerve-force to pass readily along accustomed channels.
We shall find this view of the force of habit strikingly
confirmed in a future chapter, where it will be shown that
the hair of the insane is affected in an extraordinary manner,
owing to their repeated accesses of fury and terror.
As soon as with animals the power of erection had thus been
strengthened or increased, they must often have seen the hairs
or feathers erected in rival and enraged males, and the bulk
of their bodies thus increased.
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