OUR BODILY FRAME. 31 



The mammals are the youngest and most advanced 

 class of the vertebrates. It is true they are derived 

 from the older class of amphibia, like birds and 

 reptiles ; yet they are distinguished from all the other 

 tetrapods by a number of very striking anatomical 

 features. Externally, there is the clothing of the skin 

 with hair, and the possession of two kinds of skin- 

 glands — the sweat glands and the sebaceous glands. 

 A local development of these glands on the abdominal 

 skin gave rise (probably during the Triassic period) to 

 the organ which is especially characteristic of the 

 class, and from which it derives its name — the mam- 

 marium. This important instrument of lactation is 

 made up of milk-glands (mamma) and the " mammar- 

 pouches " (folds of the abdominal skin) ; in its 

 development the teats appear, through which the 

 young mammal sucks its mother's milk. In internal 

 structure the most remarkable feature is the posses- 

 sion of a complete diaphragm, a muscular wall 

 which, in all mammals — and only in mammals — 

 separates the thoracic from the abdominal cavity ; in 

 all other vertebrates there is no such separation. 

 The skull of mammals is distinguished by a number 

 of remarkable formations, especially in the maxillary 

 apparatus (the upper and lower jaws, and the temporal 

 bones). Moreover, the brain, the olfactory organ, the 

 heart, the lungs, the internal and external sexual 

 organs, the kidneys, and other parts of the body, 

 present special peculiarities, both in general and 

 detailed structure, in the mammals ; all these, taken 

 collectively, point unequivocally to an early derivation 

 of the mammals from the older groups of the reptiles 

 and amphibia, which must have taken place, at the 

 latest, in the Triassic period — at least 12,000,000 



