This also comes from the Expression of the Emotions in Man and Animals:
Having said thus much about snakes, I am tempted to add a few remarks on the means by which the rattle of the rattle-snake was probably developed. Various animals, including some lizards, either curl or vibrate their tails when excited. This is the case with many kinds of snakes. In the Zoological Gardens, an innocuous species, the Coronella Sayi, vibrates its tail so rapidly that it becomes almost invisible. The Trigonocephalus, before alluded to, has the same habit; and the extremity of its tail is a little enlarged, or ends in a bead. In the Lachesis, which is so closely allied to the rattle-snake that it was placed by Linnaeus in the same genus, the tail ends in a single, large, lancet-shaped point or scale. With some snakes the skin, as Professor Shaler remarks, ” is more imperfectly detached from the region about the tail than at other parts of the body.” Now if we suppose that the end of the tail of some ancient American species was enlarged, and was covered by a single large scale, this could hardly have been cast off at the successive moults. In this case it would have been permanently retained, and at each period of growth, as the snake grew larger, a new scale, larger than the last, would have been formed above it, and would likewise have been retained. The foundation for the development of a rattle would thus have been laid; and it would have been habitually used, if the species, like so many others, vibrated its tail whenever it was irritated. That the rattle has since been specially developed to serve as an efficient sound-producing instrument, there can hardly be a doubt; for even the vertebrae included within the extremity of the tail have been altered in shape and cohere. But there is no greater improbability in various structures, such as the rattle of the rattle-snake,–the lateral scales of the Echis,–the neck with the included ribs of the Cobra,–and the whole body of the puff-adder,–having been modified for the sake of warning and frightening away their enemies, than in a bird, namely, the wonderful Secretary-hawk (Gypgeranus) having had its whole frame modified for the sake of killing snakes with impunity. It is highly probable, judging from what we have before seen, that this bird would ruffle its feathers whenever it attacked a snake; and it is certain that the Herpestes, when it eagerly rushes to attack a snake, erects the hair all over its body, and especially that on its tail.
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