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CHAP. IV.                     IN ANIMALS.                -            109

nocuous species, trie Goronella Sayi, vibrates its tail so
rapidly that it becomes almost invisible. The Trigono-
cephalus, before alluded to, has the same habit; and                             "I

the extremity of its tail is a little enlarged, or ends                             '/I

in a bead.   In the Laehesis, which is so closely allied                             ] i

to the rattle-snake that it was placed by Linnaeus in                             '*

the same genus, the tail ends in a single, large, lancet-                               I

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 *                         i

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                            v

would have been permanently retained, and at each                            ^

period of growth, as the snake grew larger, a new scale,                             I

larger than the last, would have been formed above it,                            , *

and would likewise have been retained.   The foundation                            I"

for the development of a rattle would thus have been                            i

laid; and it would have been habitually used, if the spe-                            }

cies, like so many others, vibrated its tail whenever it was                            l/

irritated.   That the rattle has since been specially devel-                            t

oped to serve as an efficient sound-producing instrument,                            >

there can hardly be a doubt; for even the vertebra} in-                            |*"

eluded within the extremity of the tail have-been altered                            !*

in shape and cohere.   But there is no greater improb-                            J

ability in various structures, such as the rattle of the                            -1
rattle-snake,the lateral scales of the Echis,the neck                             |

with the included ribs of the Cobra,and the whole body                            ft

of the puff-adder,having been modified for the sake                            ffr

of warning and frightening away their enemies, than in                           -11

a bird, namely, the wonderful Secretary-hawk (Gypo-                            Jl

geranus) having had its whole frame modified for the                            ft

sake of killing snakes with impunity.   It is highly prob-                           "^

able, judging from what we have before seen, that this                            "^