RANGE OF 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, R. L., "The'Reptile Book," p. 426. 



t Brown, A. E., " Texas Reptiles and their Faunal Relations," p. 558. 



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