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.
20. No two men are organized in all respects alike. There are individual variations in structure, both native and acquired. These may be too slight to be appreciable by any other test than the difference of reaction under similar external stimuli; but the variations in the sensibility to music, color, temperature, sexual influence, moral influence, etc., betray corresponding differences in the organisms. Any one variation in structure, seemingly trivial, may be the origin of well-marked diversity in physical and moral characters. Compare the bull with the ox, or the predatory aggressive eagle with the cowardly vulture. Nor are the temporary modifications to be overlooked. Antoine Cros mentions the case of a patient, a young girl, suffering from congested liver and spleen, which of course altered the state of her blood, and thus for a time modified her constitution. Her moral character was greatly altered by it. She ceased to feel any affection for father or mother; would play with her doll, but could not be brought to show any delight in it; could not be drawn out of her apathetic sadness. Things which previously had made her shriek with laughter, now left her uninterested. Her temper changed, became capricious and violent.[207] Congestion of the lungs, if unaccompanied by congestion of the liver, never produces such effects, because not thus altering the blood. The effects of liver congestion are familiar. Cros cites the case of a magistrate whose liver was enlarged, and whose skin showed a markedly bilious aspect, and in whom all affection seemed to be dead: he did not exhibit any perversion or violence, only want of emotive reaction. If he went to the theatre he could not feel the slightest pleasure in it. The thoughts of his home, his absent wife and children, were, he declared, as unaffecting to him as a problem in Euclid.