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.
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