182 PRIMITIVE FORMS OF LIFE 



were probably formed as a result of the stimulus which 

 perpetual contact with the air exercised upon the skin, and which 

 brought about, especially above the richly vascular papillae 

 of the skin, the rapid multiplication of the epidermic cells 

 which quickly dried up and accumulated outside the area 

 where they had multiplied. In this way there developed a 

 whole group of structures, with no particular object, but 

 accidentally adapted to protect the animal against loss of 

 heat. This protective apparatus, which has played the same 

 part among the Birds and the Mammals, owes its origin in 

 these two cases to entirely different circumstances. Birds, as 

 we shall see later, are only a specialized form of Reptile, itself 

 evolved from the Batrachians, whereas Mammals are directly 

 evolved from the Batrachians. Their evolution was parallel 

 with that of Reptiles, whereas the Bird, in a sense, was the 

 ultimate achievement of the evolution of the Reptile. The 

 Batrachians have a skin particularly rich in sensitive organs 

 and glands of all sorts. These glands have totally disappeared 

 in Reptiles, except in limited areas of the body, such as the edge 

 of the thigh in Lizards. The sensitive organs are equally few 

 and far between. Birds, like Reptiles, have a dry skin, and their 

 tactile organs are collected in definite regions of the body. 

 Their feathers develop like little thorns at the top of 

 dermal papillae that then become invaginated in the skin. In 

 the Mammals, on the contrary, the skin remains moist or 

 softened by numerous glands, the sudoriparous glands 

 producing sweat, and the sebaceous glands producing a liquid 

 which lubricates the hair and renders the skin oily. The hairs 

 themselves are modifications of a part of the sensitive organs of 

 the Batrachians. The bulb is often surrounded by a nervous 

 ring, turning them into tactile organs of great sensibility. 

 Instead of being formed like feathers, at the head of the 

 papillae, they arise from a deep epidermal bud which is buried 

 in the dermis or true skin and there, so to speak, indents 

 the pilary bulb. Certain skin-glands specialize in the 

 secretion of milk, and have given rise to the mammce which 

 furnish the young with their first food. The disappearance of 

 the skin-glands has made lactation impossible in Birds, whereas 

 it has become characteristic of Mammals. 



Thus the nature of their skin might seem to accentuate the 

 differences which separate Reptiles and Birds from Mammals. 



