INTRODUCTORY REMARKS. 



The present contributiou to North Americau Ilerpetology is a prodro- 

 mus of a general work on that subject, undertaken some years ago at 

 the request of the Secretary of the Smithsonian Institution. The mate- 

 rial which has been accumulating in the museum of that Institution 

 has offered great advantages for the investigation of the questions of 

 anatomical structure, variations of specillc characters, and geographical 

 distribution. It is believed that these subjects are much elucidated by the 

 study of the Batrachia and Reptilia, siuce these animals are especially 

 susceptible to physical influences; since, also, they are unable, like birds, 

 and generally not disposed, as are mammals, to make extended migra- 

 tions, their habitats express nearly the simplest relations of life to its 

 surroundings. 



In prosecuting these investigations, it has become necessary to adapt 

 the nomenclature to the results obtained by study of many specimens 

 as to the variation of species. It is a common observation that the 

 bttter a species of animal is represented in our collections, the wider do 

 we discover its range of variation to be, and the greater the number of 

 supposed distinct species does it become necessary to reduce to the rank 

 of varieties. The definition of a species being simply a number of indi- 

 viduals, certain of whose physical peculiarities belong to them alone, 

 and are at the same time exhibited by all of them, it is evident that, 

 since it is impossible, in the present state of our knowledge, to predicate 

 what those "certain peculiarities" shall be, the only test of specific defi- 

 nition is the constancy of those characters. Hence it is that the most 

 diverse forms of one species may differ more from each other than two 

 recognized species. In the investigation of North American cold-blooded 

 Vertebrata, I have observed that many species are represented by well- 

 marked geographical varieties, which, following the example of some 

 ornithologists, I have called subspecies. Many of tJiese have been here- 

 tofore regarded as species. 



In illustration of these remarks, certain species of the genus Ophibolus 

 may be selected. The most northern and the most southern forms of the 



