RANGE OP RATTLESNAKES 
131 
From the popular point of view, as Dr. Ditmars * remarks, 
the venomous rattlesnakes are the most interesting of the 
American serpents. The Old World naturalist involuntarily 
associates America with rattlesnakes ; and the ominous warn¬ 
ing sound produced by the unique appendage at their tail 
alone exercises a strange fascination on everyone who has 
become acquainted with them. Not only is the common rattle¬ 
snake (Crotalus horridus) abundant in some of the hilly por¬ 
tions of New York and Massachusetts, it actually appears to 
be increasing in numbers in these populous States, owing to 
the decrease of its natural enemies. From a zoogeographical 
point of view rattlesnakes are of considerable importance, 
because, being strictly limited to the ground, they are less 
liable to accidental dispersal than the members of many 
other genera which are expert swimmers or fond of climbing 
trees. The rattlesnakes are generally divided into two genera, 
viz., the pigmy rattlesnakes (Sistrurus) and the rattlesnakes 
proper (Crotalus). Both genera range almost all over the 
United States, a couple of species even cross the borders 
of Canada. Southward, Sistrurus is also met with in Mexico. 
The true rattlesnakes (Crotalus), on the other hand, have a 
much wider distribution in America, one species (C. terrificus) 
having been found in Mexico as well as ‘in Yucatan, Bolivia, 
Venezuela, northern Argentina, and southern Brazil. 
Remains of rattlesnakes have been noticed in a couple of the 
North American caves, otherwise they are unknown in earlier 
deposits. Their range is confined to America, and we have 
no reason to suppose that they have originated in any other 
continent. Their home is, no doubt, as Dr. Brown f suggests, 
in the south-western States in what he calls the Chihuahuan 
district. We have no means of estimating the age of the two 
genera of rattlesnakes. Yet Crotalus seems to be the more 
ancient, and, like its near relation Lachesis of southern Asia, 
Central and South America, has probably a remote ancestry. 
It may possibly have inhabited North America since early 
Tertiary times. 
I have drawn particular attention to the fact that the 
* Ditmars, E. L., “The Eeptile Book,” p. 426. 
t Bi’owu, A. E., “ Texas Eeptiles and their Faunal Delations,” p. 558. 
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