MATTIIi:\\ . ('Ll\! ATI-: \\l> FAOLUTIOK 185 



•and varied during the Mesozoic. Our knowledge of Tertiary and Quater- 

 nary birds is much more extensive, but it bears no comparison to our ac- 

 quaintance with Tertiary mammals, and the materials on which it is 

 based are for the most part very fragmentary, their identification often 

 questionable. We may say, however, that Mesozoic birds' are more com- 

 pletely kno-\vn than Mesozoic mammals ; that is to say, we know the entire 

 skeleton of two or three, and in consequence can estimate their affinities 

 more certainly and exactly. On the contrary, the fragmentary remains 

 of Cenozoic birds make our estimates of their affinities proportionately 

 uncertain and inexact. 



The Eeptilia are a more ancient class than either birds or mammals 

 and include the ancestral types of both. Our knowledge of fossil reptiles, 

 in comparison with their probable numbers and variety, past and present, 

 is much less than with mammals, more than with birds. We cannot, as 

 with Tertiary mammals, reconstruct approximate evolutionary phyla of 

 the several races from kno^vn fossil forms ; yet the evidence is sufficient to 

 give a reasonable basis for inferential phyla of some degree of exactitude 

 among many of the Mesozoic and Tertiary reptiles. But the origin of 

 the Eeptilia, like that of the Mammalia, is wrapped in obscurity, and the 

 interrelationship of the more ancient groups is a puzzle not yet solved. 

 We have a fairly extensive acquaintance with the Eeptilia of certain 

 habitats at certain epochs; but there were evidently long intervening 

 periods and important faunal facies of which we know nothing or next 

 to nothing. 



The Amphibia are not a very important group at present and are al- 

 most unknown as fossils, except for the so-called armored amphibians or 

 Stegocephalia, whose relations to the modern frogs, toads and salamanders 

 are still far from clear. This ancient group was abundant and varied in 

 Carboniferous, Permian and Triassic times and is supposed to have given 

 rise to the Eeptilia; but the relationship has not been satisfactorily dem- 

 onstrated by fossils, nor is there direct evidence of the interrelationship 

 of the several groups of stegocephalians. 



A wide gap separates the oldest four-footed vertebrates from any known 

 fishes, living or extinct. 



Zoological Eegions, Past and Present 



The zoological divisions of the land surface of the earth are given by 

 Lydekker^^ as follows : 



13 Richard Lydekker : A Geographical History of Mammals. 1896. This is a modifi- 

 ■c-atlon of the regions proposed by Selater in IS.'jS (.Tour. I'roc. Linn. Soc. vol. ii. pp. 130- 

 146) and aduptod by Wallace in ISTf! (Geographical Distribution of Animals). 



