According to these tests, what class of vertebrates should _rank next to mammals_? Compare the heart, lungs, blood, and parental devotion of these two highest classes of animals. [Illustration: FIG. 339.--SKELETON OF LION (cat family).] =The first mammals=, which were somewhat like small opossums, appeared millions of years ago, when the world was inhabited by giant reptiles. These reptiles occupied the water, the land, and the air, and their great strength and ferocity would have prevented the mammals from multiplying (for at first they were small and weak), but the mammals carried their young in a pouch until able to care for themselves, while the reptiles laid eggs and left them uncared for. The first mammals used reptilian eggs for food, though they could not contend with the great reptiles. Because birds and mammals are better parents than reptiles, they have conquered the earth, and the reptiles have been forced into subordination, and have become smaller and timid. [Illustration: FIG. 340.--WALRUS (_Trichechus rosmarus_).] =Classification of Mammals.=--Which two have the closest _resemblances_ in the following lists: Horse, cow, deer. Why? Cat, cow, bear. Why? Monkey, man, sheep. Why? Rat, monkey, squirrel. Why? Giraffe, leopard, camel. Why? Walrus, cat, cow. Why? Check the five mammals in the following lists that form a group _resembling each other most closely_: Lion, bear, pig, dog, squirrel, cat, camel, tiger, man. State your reasons. Giraffe, leopard, deer, cow, rat, camel, hyena, horse, monkey. State reasons. [Illustration: FIG. 341.--WEASEL, in summer; in Canada in winter it is all white but tip of tail.]