184 ANNALS NEW YORK ACADEMY OF SCIENCES 



tliirds of the evolutionary history of the Mammalia. The rest is theory 

 and hypothesis. 



Assuredly, we have no right to assume that the few species which have 

 been founded upon these fossil remains represent at all adequately the 

 number and variety of mammals that lived during the Mesozoic; nor can 

 we even suppose that they fairly represent them. Only two^- of the 

 numerous phyla of early Tertiary mammals can be at all directly derived 

 from known Mesozoic ancestors. The rest are descended from unknown 

 forms. We may suppose, from the evidence at hand, that the known 

 Jurassic and Cretaceous mammals were arboreal swamp-<lwellers and that 

 the chief reason why wc know so little of the Mesozoic mammals is that 

 the deposits of the upland regions where they chiefly lived have not been 

 conserved to our day, or at all events have not been recognized and suffi- 

 ciently explored for fossils. 



In the Tertiary, mammals suddenly spring into (apparent) promi- 

 nence, mainly, it may be assumed, because the fluviatile and eolian forma- 

 tions of the Cenozoic still exist in many localities, although they are being 

 rapidly eroded and carried down to the coastal swamp and sea margin 

 areas of deposition. Epicontinental deposits of Eocene age are rare and 

 scattered, and our knowledge of Eocene mammals is obtained from only 

 a few localities and largely from fragmentary specimens. Through the 

 following Tertiary epochs, these deposits become progressively more ex- 

 tensive and abundant, and our knowledge of fossil mammals is corre- 

 spondingly greater. Finally, in the Quaternary, they form a mantle over 

 most of the earth's surface, and the fossil mammals are so well known 

 and so many specimens from so many localities have l)een found that we 

 can get a fairly accurate idea as to the range of many species, not merely 

 as discQvered in one or another continent, but as to wliat parts of that 

 continent they inhabited. 



If our knowledge of fossil mammals is incomplete, that of fossil birds 

 is very much more fragmentary. They probably came into existence at 

 about the same time as mannnals, l)ut the early stages of their evolution 

 are even more obscure, and comparison of the living members of the class 

 affords less evidence than with mammals as to their source and course of 

 progress. They are even rarer than mammals in tlie Mesozoic. Two 

 skeletons and a feather from the Jurassic of Bavaria, a number of skele- 

 tons and fragments from the late Cretaceous of Kansas and a few frag- 

 ments of the skeleton from Cretaceous formations in New Jersey and 

 Europe, — these are all we know of a class Avhicli was prnliaMy very large 



^- Plagiaulacidse and Dldelphyidae. 



