Chap. IV. IN ANIMALS. 109 



nocuous species, the Coronella Sayi, vibrates its tail so 

 rapidly that it becomes almost invisible. The Trigono- 

 cephaly, 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 spe- 

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

 irritated. That the rattle has since been specially devel- 

 oped to serve as an efficient sound-producing instrument, 

 there can hardly be a doubt; for even the vertebras in- 

 cluded within the extremity of the tail have been altered 

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

 abilitv 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 (Gypo- 

 geranus) having had its whole frame modified for the 

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

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



