19. A word on each. There is a structural disposition of the parts which is common to large groups of organisms, so that a corresponding similarity is observable in the reactions of these organisms. Thus all quadrupeds use their limbs for locomotion in very similar ways; birds use their wings for flight in similar ways. All vertebrates swallow their food, defend themselves, shrink when hurt, etc., in ways that are very similar. In so far as their organizations are alike, their actions and reactions are alike. In so far as their organizations differ, their actions and reactions differ. The goose and the vulture are alike in the main lines of structure; still more alike are duck and hen; yet, owing to certain unlike characters of structure, they manifest some marked differences in action and reaction: the goose will starve in the presence of food which the vulture gluttonously devours, and the vulture will refuse the vegetable food which the goose devours; the duck plunges into the water, the hen not only refuses to enter it, but is greatly agitated when she sees the ducklings she has hatched plunging into it. That peculiar instincts, habits, and feelings are rigorously determined by peculiarities in the organism, no one doubts, when animals are in question. If this is less obvious in the case of men, the reason is that there the influence of other factors somewhat masks the operation of the primary constitution--these factors are the modified and the temporary constitutions. Yet even in man it is true to say that his feelings and actions are the result of his organization, native and acquired.