Page:Prehistoric Britain.djvu/27

Rh supply of food than was possible in a fixed position, such as that of a plant. Among lower organisms movement was accomplished in various ways—vibratile filaments, ciliary organs, pseudopodia, etc.; but in all the higher vertebrates locomotion was effected by means of four movable limbs, capable of supporting and transporting the animal at will. As these four-footed animals became greatly affected by the struggle for life, owing to the rapid multiplication of species and the ever-varying conditions of the environment, it followed that the limbs became also more or less modified, so as to make them suitable, not only for increased speed in altered circumstances, but useful to the animal economy in other ways. Hence they became adapted for diverse purposes, such as swimming, flying, climbing, grasping, scraping, etc. The anterior limbs, owing to their proximity to the head, were more frequently selected for such transformations, as may be seen in the wings of birds and bats. But whatever modifications the fore-limbs may have undergone, no animal, but man, has ever succeeded in divesting them of their primary function of locomotion. This achievement was primarily due to the attainment of the erect attitude, which necessitated a rearrangement of the functions of the limbs—the anterior being henceforth entirely restricted to manipulative and prehensile purposes, and the posterior to locomotion.

The maintenance of the erect attitude