12 PROCEEDINGS OF THE ACADEMY OF [Jan., 



The chief purpose of the present paper is to inquire into the 

 nature of these variations, and to determine if possible how far 

 they are promiscuous and without meaning, or to what extent they 

 may be beheved to fall within those ideas of progressive modifica- 

 tion, without which as a guiding principle, the practice of tax- 

 onomy is mere byplay. The conception which has directed the 

 inquiry, is that a relatively high degree of constancy and isolation 

 is essential to the recognition of a species ; and that variations, to 

 be of subspecific value, must be of such a character as to offer 

 reasonable grounds for the belief that they are stages of change; 

 an important pai't of such character being that they shall occur in 

 sufficient numbers to constitute centres, so to speak, upon lines 

 leading from established forms. 



The color descriptions have in a large proportion of cases been 

 taken from the living snake ; to which it may be added that in 

 addition to the alcoholic series to which I have had access, includ- 

 ing that of the Academy of Natural Sciences of Philadelphia, 

 which now contains the whole private collection of the late Prof. 

 Cope, nearly four thousand living specimens of North American 

 snakes have passed under my observation, in the course of identify- 

 ing the large amount of material in this group which comes into 

 possession of the Zoological Society of Philadelphia. 



The more recent works w'hich treat with modern methods, of the 

 whole field of North American snakes, are the paper of Prof. Cope, 

 above referred to,"* and Mr. G. A. Boulenger's Catalogue of Snakes 

 in the British Museum (1893-96). 



With neither of these distinguished naturalists am I able to find 

 myself in full accord; the one appearing to mc to err in excess of 

 analysis, quite as much as the other does in the opposing method. 



The literature has been so fully worked out by Mr. Boulenger, 



■* Since the completion of the present paper (October, 1900), the E^po^•t of 

 the U. S. Natiorud MuseurnloT 1898 has appt^ared. containing Prof. Cope's 

 posthumous work on North American sealed reptiles (Serpentes, pp. G83~ 

 1198). I find that few of the conclusions which I bad reached are thereby 

 altered, for the chief additions to his previous paper of 1892 result from the 

 introduction of characters drawn from the male generative organ. Tiiese 

 were not made nse of in the present paper, for examination of much of 

 Cope's material, and some further investigations of my own, had satisfied 

 me that much verification and exteasi(ni remains to be done before their 

 valne in generic determination can be established. Such changes as I have 

 made have been introduced into the text, and references to the paper are 

 given as "Cope, Kep. Nat- Mu'^.,'" "Cope, /.c," indica'^ingthe previous paper, 

 above cited. 



