640 MAMMALIA. 



natural selection. But it is important to recognise that the 

 maternal sacrifice whatever its origin may have been 

 expresses a subordination of self-preserving to species-main- 

 taining. Thus, other-regarding as well as self-regarding 

 activities have been factors in evolution. 



History of Mammals. As to the origin of Mammals we can only 

 speculate. There are some remarkable resemblances between Mono- 

 tremes and certain extinct Reptilian types, known as Anomodontia or 

 Theromorpha, and these again exhibit affinities with the extinct Laby- 

 rinththodont Amphibians. Amphibians and Mammals agree in having 

 two occipital condyles, small quadrates, large squamosals, and in certain 

 characteristics of pectoral and pelvic girdles. Possibly the ancestral 

 Mammals and the Anomodont Reptiles diverged from a common 

 Amphibian stock. 



The oldest known remains of Mammals are some fossils from Triassic 

 rocks, and similar types have been found in Cretaceous and Jurassic 

 beds ; most of these Mesozoic fossils are but small pieces of small 

 animals, and secure conclusions as to their nature are not readily 

 reached. The earliest suggest affinities with Marsupials and Insecti- 

 vora. Many of the Mesozoic mammals belong to a group which has 

 received the name of Multituberculata, on account of the longitudinal 

 rows of tubercles on the back teeth. It is possible that these forms, e.g., 

 PlagiaulaX) Tritylodon, Poly mastodon, should be ranked beside the 

 Monotremes, but they are usually placed nearer Marsupials. 



Other Mesozoic forms, such as Dromatheritim, Triconodon, Amphi- 

 therium, Spalac other itim, are often referred to the Marsupial series 

 beside opossum, dasyure, and bandicoot. 



The first certain remains of Placental Mammals are found in Eocene 

 strata, and give evidence of the existence of generalised types connecting 

 rather than referable to the modern orders. Many are characterised by 

 the presence of three tubercles on the back teeth, and of five digits on 

 the limbs, and by having brains relatively smaller than those of their 

 modern successors. 



Among extinct Tertiary types, we may especially notice the 

 ground sloths (e.g., Megatherium] and Glyptodonts allied to the 

 modern Edentata, the Zeuglodonts, sometimes included among 

 Cetaceans, numerous ancestral Ungulates, and the Creodonts allied 

 to modern Carnivores. 



More detailed account of some of the structures of Mammals. 

 Skin. 



The skin consists of a superficial epidermis derived from 

 the outer or ectodermic layer of the embryo, and of a sub- 

 jacent mesodermic dermis or cutis. 



The most characteristic modification of the mammalian 

 epidermis is the hair. Each hair arises from the cornifica- 



