THE RATTLESNAKE AND NATURAL SELECTION. 35 
is by no means so easy, even if we allow all that can be claimed for 
natural selection, to account for the development of this ap- 
pendage. The following seems to me the most satisfactory con- 
ception of its evolution, looking at the matter from Mr. Darwin’s 
point of view. It is a fact well known, doubtless, to those who 
have observed serpents, though I find no mention of it in the 
works I have consulted, that many serpents, when in a state of 
excitement vibrate the end of their tail just as the rattlesnake 
does.* This movement is likely enough the same in character as 
that which occurs in the hinder part of the spinal column 
among higher animals under excitement. The wagging of the 
dog’s tail is a rhythmical movement of essentially the same 
character as the movement of the rattlesnake. Taking the same 
line of argument as that adopted by Mr. Darwin with regard to 
the monthly phenomena observable among the mammalia, it could 
be claimed that the tendency to move the tail was explicable on 
the following grounds. During more than half the lifetime of the 
group of vertebrates, from the point of their presumed origin at 
the close of the Silurian down to the present day, the caudal por- 
tion of the body was used as the propelling agent. Fishes, with 
slight exceptions, propel themselves by a reciprocating movement 
of the tail. All conditions of excitement at once manifest them- 
selves in the violent movement of this part of the body. Whether 
in flight or chase or under the influence of sexual excitement, this 
movement is the important element of success. It is by no means 
surprising that the motion which was for ages the point which natua- 
ral selection operated most intensely, for those forms which had 
the capacity for making this alternate movement of the tail with 
the greatest rapidity would be most successful in flight or chase, 
should have survived its usefulness and remained as a mere feature 
of expression in most of our animals. It may be remarked in 
rattles of the tail. In this genus, therefore, there could have been no advantage derived 
— openness It m may be said, ee that the ‘rates which have little fanctional 
+ 
number and shape than in the Crotalus. Tt 
tendency to form rattles in this group of serpents and these structures are seized upon 
tural selection and made functional. 
Man the above matter was as put in type, I pare learned that Prof. Jeffries Wyman 
ry; ema the occur- 
rence of this Ripon in the tails of sakes: oebee than than the rattlesnake, some two or 
ago. any notice of the Sp siahcidtations in the Proceed- 
three years e fail 
- ings of the Sects. sùgh there can be no doubt that this eminen- naturalist should 
bec the priority on this point, 
