INTRODUCTION 



Class II. Mammalia 



The mammals are as sharply marked by their hair as are the 

 birds by their feathers, lower jaw suspended directly from the skull. 

 They have warm blood; except the monotremes they bring forth 

 living young; the young of all are nourished by milk secreted by 

 glands (mammae) in the mother. 



There are a few other terms of convenience which may be defined 

 here as they will save much circumlocution. The term Teleostomes 

 is applied to ganoids and teleosts, from the fact that they have true 

 jaws. The amphibia and the amniotes are frequently united as 

 Tetrapoda, from their possessing feet, in contrast to the fishes with 

 fins. 



The geological history of these groups is important; their first 

 appearance and their geological range is indicated in the accompany- 

 ing table of the geological periods. 



INTRODUCTORY EMBRYOLOGY *^^ 



The structure of an adult vertebrate can be fully appreciated and 

 the bearing of the facts recognized only by a knowledge of the develop- 

 ment of the parts concerned. It 

 would often appear, for example, 

 that certain organs in different 

 groups were exact equivalents of 

 each other — duplicates in function 

 and details of structure — while a 

 knowledge of their development 



may show that they have had en- 

 FiG. 2. — Successive Stages in the seg- . . . . .„ 



mentation of an amphibian egg. 1-7, tirely different origms and differ- 

 Results of the corresponding cleavage ^^t histories, and hence cannot 



planes. ' 



be identical; they are examples of 

 what the evolutionist calls convergent evolution. Such cases are 

 apt to lead one astray as to the relations of the forms in which they 

 occur. Farther, the development affords a framework around which 

 Ipthe details of organization may be arranged in a logical manner, 

 thus ading in their remembrance. For these reasons the following 

 pages are based on embryology Not only are the histories of the 

 separate organs traced before an account is given of the adult con- 

 ditions but this introductory chapter gives in the most generalized- 

 form the earlier stages before the organs are outlined. 



