36 THE RATTLESNAKE AND NATURAL SELECTION. 
passing that the obstinate aniya of the tail among the verte- 
brates may be accounted for by the intense bodily inertia, if we 
may so call it, which causes the energy of survival of useless — 
structures to be proportionate to the length of time which they — 
have been of use to the groups to which they belong. It is natural : 
enough that a part of the body situated at one of the regions of ; 
manifold relations as the tail is, and unappropriated to any spe — 2 
cial function, should be put to use in various ways, as a prehensile : 
instrument by some monkeys and other animals, or a building tool ‘ 
by the beavers, as a fly-brush by many others, etc. 1 
Mr. Herbert Spencer has already suggested that the wagging ¢ of 
_ the dog’s tail and similar movements of that appendage is in fact an q 
escape of nervous force restrained from other modes of expression — q 
at the moment. Looking at the matter from this point of view, 
which is doubtless quite satisfactory, we may reconcile it perfectly 
with the views which have just been presented by supposing that 
the ancient and no longer functional channel of escape for nervous 
force, the tail, has remained the way of outlet for the suppressed 
energy of the animal. The older the channel the less easy iti 
to close it either by volition or by natural selection. 
Be the cause of the persistence of the tail and its movem 3 
what it may, we are still justified in assuming as the starting i 
motion of the tail common among snakes. It is the opinion 0 
some herpetologists that the rattles are the remains of the skins | 
successively shed by the animal. The rate of development of 
rattles, together with the fact that the skins of some serpents 
more imperfectly detached from the region about the tail than 
_ other parts of the body, makes this view very probable. Let 
suppose that we had a group of poison-fanged serpents, accid 
tally tending to keep the tail skin in the peculiar fashion of ratt 
snakes and that in some of these it was persistent enough to ma 
the whirring sound of the Cicada when the tail was rapidly mo 
under excitement. These would survive and breed the most surely 
and so that feature would become hereditary. The great varjab 
ity in the number of rattles in the different forms of rattles? 
and the late time of their development, even among those wW% 
differ in no other regard, would seem to indicate that this struc? 
has not yet been firmly fixed by long inheritance. 
The reader will please not suppose that because I have | pole 
