14 WATER REPTILES OF THE PAST AND PRESENT 



which they were printed. When facts are numerous enough, con- 

 clusions are patent even to the novice; when facts are few and 

 obscure, one can guess about as well as another. In general, it may 

 be said that the older a group of animals is the more abstruse are 

 the problems presented; first, because of the lack of abundant 

 material; second, because the forms speak to us in an unfamiliar 

 language that we cannot easily interpret. The classification of the 

 mammals approaches more nearly the ultimate truth than does that 

 of any other group of organisms, because we know more about the 

 extinct forms than we do of any other class, and also because we 

 know more about the living forms than we do about any other 

 living animals. 



Species of reptiles are, for the most part, vague quantities in 

 paleontology; they can be determined with assurance only by the 

 comparison of abundant material. Adult characters in mammals 

 are apparent in the ossification of the skeleton, and size can be used 

 within moderate limits in the determination of species; but size 

 in reptiles means but little; no one could possibly say that the 

 skeleton of an alligator six feet in length is not that of an adult 

 animal if he knew nothing else about the Crocodilia. So also the 

 compression and malformations of bones from the processes of 

 fossilization obliterate specific characters in great part. Nor are 

 specific characters easily distinguishable in the skeletons of living 

 reptiles. The genus, therefore, among fossil reptiles is practically 

 the unit, and we may be sure that for every well-defined genus we 

 discover there existed numerous minor variations, which, had we 

 the living animals to study, we should call species. We classify 

 the living Crocodilia into two families, about four well-defined 

 genera — perhaps even five or six — and about twenty-five species. 

 Of the living lizards there are about eighteen hundred species, 

 twenty families, and four larger groups or suborders. In all 

 probability the lizards have never been more abundant and more 

 varied than they are at the present time. Possibly these propor- 

 tions of species, genera, families, and suborders may represent 

 approximately the proportions that have existed at some time or 

 other in most of the other groups which we call orders — approxi- 

 mately only, for we can never be quite sure that we evaluate the 



