THE SKIN AND THE HAIR. 533 



Amphitherium, Spalacotherium, are often referred to the Mar- 

 supial 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 included 

 among Cetaceans, numerous ancestral Ungulates, and the 

 Creodonts allied to modern Carnivores. 



More detailed account of some of the structures 

 of Mammals. 



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- 

 tion of an ingrowing epidermic papilla, surrounded at its 

 base by a moat-like follicle, and nourished during growth 

 by a vascular projection of the dermis. 



Each hair consists of a spongy central part and a denser cortex, but 

 there are many diversities of form and structure, such as short fur and 

 long tresses, the soft wool of sheep and the bristles of pigs, the spines of 

 hedgehog, porcupine, and echidna, the cilia of the eyelids and the tactile 

 vibrissse of the lips and cheeks. 



The hair keeps the animal dry and warm ; in the practically hairless 

 Cetacea the layer of fat or blubber underneath the skin also serves to 

 sustain the temperature of the body. Like feathers, hairs die away 

 and are cast off, being replaced by fresh growths. A few mammals, 

 such as the Arctic fox, the mountain hare, and the ermine, become white 

 in winter, harmonising with the snow. In the case of Ross' lemming, 

 we know that this change is directly due to the influence of the cold, 

 and depends iri great part on the appearance of gas-bubbles inside the 

 hairs. 



That the colouring is sometimes of protective advantage, we have 

 already noticed ; but in many cases no utilitarian interpretation can b'e 

 read into the stripes and markings. Those of related species often form 

 regular series, and are superficial outcrops of constitutional changes 

 hardly to be analysed. Sometimes there is considerable change during 



