21. Owing to the recognized dependence of peculiar instincts and modes of reaction on peculiarities of structure, comparative anatomists are quite confident, when they find a portion of a skull with two occipital condyles, that the animal to which this skull belonged had red blood-corpuscles without nuclei, and (if a female) suckled its young. If in that fragment of skull there remain a single tooth, it will prove that the animal was carnivorous or herbivorous, and had, or had not, retractile claws. From such data a general conclusion may be formed as to the instincts and habits of the animal. The data disclose much of the primary constitution, that is to say, the mechanism which the animal brought with it into the world, ready prepared to react in definite ways on being stimulated. The connate mechanism has correlative tendencies of reaction. Some of these tendencies are inevitably called into play by external conditions, and they continue unaltered amid great varieties of circumstances, provided none of these variations directly deprive them of their appropriate stimulation. Such tendencies of the connate mechanism are styled _automatic_ (an unfortunate metaphor, which has led to the theory of Automatism), and include, besides the visceral reactions, the more complex reactions of winking, breathing, swallowing, coughing, flying, walking, etc. It is true that we learn to walk, and learn to wink, whereas the other actions require no tentative efforts directed by experience; but the mechanism of all these actions is already laid down in the primary constitution, and is inevitably called into play.