Fig. 101.—The King Penguin. A highly specialised diving bird.
Photo: W. P. Dando.
CHAPTER VI
THE MAMMALS AND MAN
The subject of our discussion is now narrowed down to the group of the mammals. The mammals are characterised by two very obvious features: a body-covering of hair, and a set of special glands in the female which secrete milk for the nourishment of the young. These are constant characters, and neither is ever found in any other group. As to how the hair originated, nothing definite is known; but (while there are certain difficulties in regard to the theory) it is on the whole reasonable to suppose that the mammalian hair arose, as the bird's feather undoubtedly did, as a modification of the reptile's scale. The mammary gland appears to represent a modification of other skin glands, either of sweat glands or more probably of the oil glands which exist in connection with the hairs.
Another important character, already mentioned at the end of the last chapter, is the diaphragm, a muscular partition separating the body cavity into a thoracic and an abdominal portion. The diaphragm has important functions in connection with the mechanism of breathing. By means of it the thoracic cavity can be increased or diminished in size, and air thus drawn into or expelled from the lungs. It is interesting to observe that a similar partition, with the same function, occurs in the Crocodiles, but this has different relations to the abdominal organs, and has evidently evolved quite independently.