120 



THE THEORY OF EVOLUTION 



For the tabulation of subdivisions (classes of the 

 family) no generally available criteria can be given. 

 A textbook of geology should be read in that connec- 

 tion ; as concerns systematic arrangements Selenka's 

 ' Zoologisches Taschenbuch ' is the best. 



We will now give an example from the last-named 

 work. 1 Many of the characters described must be 

 simply taken as ' given/ They contribute much to the 

 total habit, but we cannot speak of higher or lower 

 forms. 



Typical Differences between Reptiles and Mammalia. 

 KEPTILES. MAMMALIA. 



Skin scaly, offering no pro- 

 tection against cold ; temperature 

 changeful (Poikilotherm). 



Skin protected from cold by 

 hairy coat or cushion of fat ; 

 temperature even and high. 



Larger and smaller circulation, Perfectly separated; active 



mostly and imperfectly sepa- assimilation, 

 rated ; slow assimilation. 



Lungs sack-like, of small super- Lungs alveolar, with large 



ficies reaching to the abdomen ; surface confined by diaphragm 



chest small. (breathing muscle) in thorax ; 



chest broad. 



Mainly dependent on animal Manifold provision, hence dis- 



food and warm climate. The tribution more extended. The 



ovum is developed by aerial ovum is hatched by maternal 



temperature. warmth. 



Teeth alike ; persistent change 

 of teeth (polyphyodont). Vege- 

 tarians are monophyodont or 

 anodont (Tortoises). 



1 Selenka : Zoologisches Tascheiibuch, I, p. 190. 



Teeth dissimilar ; stomach re- 

 lieved by grinding of food ; jaw 

 typically diphyodont. 



