feathers is almost always accompanied by various voluntary movements by
threatening gestures, opening the mouth, uncovering the teeth,
spreading out of the wings and tail by birds, and by the utterance of
harsh sounds; and the purpose of these voluntary movements is
unmistakable. Therefore it seems hardly credible that the co-ordinated
erection of the dermal appendages, by which the animal is made to
appear larger and more terrible to its enemies or rivals, should be
altogether an incidental and purposeless result of the disturbance of
the sensorium. This seems almost as incredible as that the erection by
the hedgehog of its spines, or of the quills by the porcupine, or of
the ornamental plumes by many birds during their courtship, should all
be purposeless actions.
We here encounter a great difficulty. How can the contraction of the
unstriped and involuntary _arrectores pili_ have been co-ordinated with
that of various voluntary muscles for the same special purpose? If we
could believe that the arrectores primordially had been voluntary
muscles, and had since lost their stripes and become involuntary, the
case would be comparatively simple. I am not, however, aware that there
is any evidence in favour of this view; although the reversed
transition would not have presented any great difficulty, as the
voluntary muscles are in an unstriped condition in the embryos of the
higher animals, and in the larvae of some crustaceans. Moreover in the
deeper layers of the skin of adult birds, the muscular network is,
according to Leydig,[420] 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
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