CHAPTER XXV 



SUB-PHYLUM IV. CRANIATA 



DIVISION II. GNATHOSTOMATA 



SUB-DIVISION II. AMNIOTA 



Class V. MAMMALIA 



THE class Mammalia (Lat. mammae, breasts), the last division 

 of the phylum Vertebrata, includes those animals 



General * J 



character. which suckle their young. Like the Birds, their 

 temperature is constant and they have the ventricle 

 of the heart completely divided into two halves. But they differ 

 from Birds in never possessing feathers; only in one order is the 

 fore-arm converted into a wing, and even in this case the arrange- 

 ment of the parts is quite different from that in the Bird's wing. 



Besides these characters however there are a large number of 

 others in which, while Mammals differ from both Birds and Reptiles, 

 the last-named two groups agree with one another, so that for a 

 long time the opinion was held that Mammals were vastly further 

 removed from Reptiles than were Birds ; and indeed if only modem 

 Reptiles were considered this could not well be denied. If however 

 we examine the remains of the Reptiles which have existed on the 

 earth in past time, we come to the conclusion that the better way 

 to state the difference would be to say that, whereas Birds might be 

 traced back to Reptiles not very unlike modern lizards, Mammals 

 are derived from a type which has died out, leaving no modern 

 representatives. Thus Mammals are almost certainly descended 

 from the extinct group Theromorpha and birds from some Rhyn- 

 chocephalan ancestor. 



Just as feathers constitute an indubitable mark of a Bird, so 

 true hairs are equally characteristic of Mammals. It is true that 

 the word hair is loosely used, being often applied for instance to the 

 delicate flexible spines of caterpillars, which are constructed on a 

 totally different plan to the hairs of Mammals. A hair in the 

 zoological sense is a rod composed of closely packed cells converted 

 into horn, and under a microscope the outline of these cells can be 



