THE EVOLUTION OF THE MAMMALIA 453 



increase its sensitiveness. The hair covering, furthermore, 

 was a non-conductor of heat, and this fact together with the 

 greater activity of the animal and more intense metabolism 

 enabled the mammals to become warm-blooded. Later on, 

 with the development of the sweat-glands in the skin, the 

 mammals were able to regulate their loss of heat, and so become 

 constant- temperatured or homothermous. The modification 

 of some of the skin-glands into mammary glands made it 

 possible for the young mammals to pass through a protected 

 period of infancy during which the finishing touches to their 

 development were put on, and they became apprenticed 

 under the care of the family to the conditions of their adult 

 life. 



The transition from Theromorph reptiles to mammals 

 probably took place in the Permian period, for in the Triassic, 

 fossils are found which show an advance in grade of structure. 

 Of these, the Multituberculata are a group which persisted 

 until the Eocene. They advanced in general evolution and 

 grade of structure as far as the Marsupials. The pelvis was 

 narrow as in the reptiles, and the lower jaw which contained 

 a single bone, had inflected angles. The single bone (dentary) 

 in the lower jaw is a characteristic mammalian feature. The 

 Multituberculata were, however, specialised, and possessed 

 molar teeth with a large number of cusps. They are probably 

 a divergent line which evolved parallel with but independently 

 from the remaining mammals. 



At this stage it must be imagined that the primitive mam- 

 mals had seven cervical vertebrae as a constant number, and 

 that they had evolved the characters enumerated above, 

 together with the diaphragm and the non-nucleated red blood- 

 corpuscles. The epipterygoid had been converted into the 

 alisphenoid, and the quadrate (incus) and articular (malleus) 

 into auditory ossicles. They retained, however, the reptilian 

 characters of the presence of the coracoid, precoracoid, and 

 interclavicle, the cloaca and the habit of laying eggs. They 

 had not yet evolved the viviparous habit or the formation of a 

 placenta, epiphyses were not yet well developed in the bones, 

 the mammary glands were unprovided with teats, and the two 



