The males of some lizards, when fighting together during their courtship, expand their throat pouches or frills, and erect their dorsal crests.[16] But Dr. Gunther does not believe that they can erect their separate spines or scales.

We thus see how generally throughout the two higher vertebrate classes, and with some reptiles, the dermal appendages are erected under the influence of anger and fear. The movement is effected, as we know from Kolliker's interesting discovery, by the contraction of minute, unstriped, involuntary muscles,[17] often called _arrectores pili_, which are attached to the capsules of the separate hairs, feathers, &c. By the contraction of these muscles the hairs can be instantly erected, as we see in a dog, being at the same time drawn a little out of their sockets; they are afterwards quickly depressed. The vast number of these minute muscles over the whole body of a hairy quadruped is astonishing. The erection of the hair is, however, aided in some cases, as with that on the head of a man, by the striped and voluntary muscles of the underlying _panniculus carnosus_. It is by the action of these latter muscles, that the hedgehog erects its spines. It appears, also, from the researches of Leydig[18] and others, that striped fibres extend from the panniculus to some of the larger hairs, such as the vibrissae of certain quadrupeds. The _arrectores pili_ contract not only under the above emotions, but from the application of cold to the surface. I remember that my mules and dogs, brought from a lower and warmer country, after spending a night on the bleak Cordillera, had the hair all over their bodies as erect as under the greatest terror. We see the same action in our own _goose-skin_ during the chill before a fever-fit. Mr. Lister has also found,[19] that tickling a neighbouring part of the skin causes the erection and protrusion of the hairs.

[15] _Melopsittacus undulatus_. See an account of its habits by Gould, `Handbook of Birds of Australia,' 1865, vol. ii. p. 82.

[16] See, for instance, the account which I have given (`Descent of Man,' vol. ii. p. 32) of an Anolis and Draco.

[17] These muscles are described in his well-known works. I am greatly indebted to this distinguished observer for having given me in a letter information on this same subject.

From these facts it is manifest that the erection of the dermal appendages is a reflex action, independent of the will; and this action must be looked at, when, occurring under the influence of anger or fear, not as a power acquired for the sake of some advantage, but as an incidental result, at least to a large extent, of the sensorium being affected. The result, in as far as it is incidental, may be compared with the profuse sweating from an agony of pain or terror. Nevertheless, it is remarkable how slight an excitement often suffices to cause the hair to become erect; as when two dogs pretend to fight together in play. We have, also, seen in a large number of animals, belonging to widely distinct classes, that the erection of the hair or 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.

[18] `Lehrbuch der Histologie des Menschen,' 1857, s. 82. I owe to Prof. W. Turner's kindness an extract from this work.

[19] `Quarterly Journal of Microscopical Science,' 1853, vol.

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Charles Darwin

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