It should be clear to any one accustomed to the working of biological laws that all the tendencies of a living organism are progressive in their development, and are held in check by the interaction of their several forces. Each living form, with its dominant characteristics, represents a balance of power, a sort of compromise. The size of earth’s primeval monsters was limited by the tensile strength of their material. Sea monsters can be bigger, because the medium in which they move offers more support. Birds must be smaller for the opposite reason. The cow requires many stomachs of a liberal size, because her food is of low nutritive value; and she must eat large quantities to keep her machine going. The size of arboreal animals, such as monkeys or squirrels, is limited by the nature of their habitat: creatures that live in trees cannot be so big as creatures that live on the ground. Every quality of every creature is relative to its condition, and tends to increase or decrease accordingly; and each quality tends to increase in proportion to its use, and to decrease in proportion to its disuse. Primitive man and his female were animals, like other animals. They were strong, fierce, lively beasts; and she was as nimble and ferocious as he, save for the added belligerence of the males in their sex-competition. In this competition, he, like the other male creatures, fought savagely with his hairy rivals; and she, like the other female creatures, complacently viewed their struggles, and mated with the victor. At other times she ran about in the forest, and helped herself to what there was to eat as freely as he did.