Why does the author give a definition of instinct

单项选择题

Instinct is usually defined as the faculty of acting in such a way as to produce certain ends, without foresight of the ends, and without previous education in the performance. That instincts, as thus defined, exist on an enormous scale in the animal kingdom needs no proof. They are the functional correlatives of structure. With the presence of a certain organ goes, one may say, almost a native aptitude for its use. "Has the bird a gland for the secretion of oil She knows instinctively how to press the oil from the gland, and apply it to the feather. Has the rattlesnake the grooved tooth and gland of poison He knows without instruction how to make both structure and function most effective against his enemies. Has the silk worm the functions of secreting the fluid silk At the proper time she winds the cocoon such as she has never seen, as thousands before have done; and thus without instruction, pattern, or experience, forms a safe abode for herself in the period of transformation. Has the hawk talons She knows by instinct how to wield them effectively against the helpless quarry. " (Chadbourne, 1872) A very common way of talking about these admirable definite tendencies to act is by naming abstractly the purpose they subserve, such as self-preservation, or defence, or care for eggs and young—and saying the animal has an instinctive fear of death or love life, or that she has an instinct of self-preservation, or an instinct of maternity and the like. But this represents the animal as obeying abstractions which not once in a million cases is it possible it can have framed. The strict physiological way of interpreting the facts leads to far clearer results. The actions we call instinctive all conform to the general reflex type; they are called forth by determinate sensory stimuli in contact with the animal’s body, or at a distance in his environment. The cat runs after the mouse, runs or shows fight before the dog, avoids falling from walls and trees, shuns fire and water, etc. , not because he has any notion either of life or of death, or of self, or of preservation. He has probably attained to no one of those conceptions in such a way as to react definitely upon it. He acts in each case separately, and simply because he cannot help it; being so framed that when that particular running thing called a mouse appears in his field of vision he must pursue; that when that particular baking and obstreperous thing called a dog appears there he must retire, if at a distance, and scratch if close by; that he must withdraw his feet from water and his face from flame, etc. His nervous system is to a great extent a preorganized bundle of such reactions—they are as fatal as sneezing and as exactly correlated to their special excitants as it is to its own. Although the naturalist may, for his own convenience, class these reactions under general heads, he must not forget that in the animal it is a particular sensation or perception or image which calls them forth. At first this view astounds us by the enormous number of special adjustments it supposes animals to possess readymade in anticipation of the outer things among which they are to dwell. Can mutual dependence be so intricate and go so far Is each thing born fitted to particular other things, and to them exclusively, as locks are fitted to their keys Undoubtedly this must be believed bo be so. Each nook and cranny of creation, down to our very skin and entrails, has its living inhabitants, with organs suited to the place, to devour and digest the food it harbors and to meet the dangers it conceals; and the minuteness of adaptation thus shown in the way of. structure knows no bounds. Even so are there no bounds to the minuteness of adaptation in the way of conduct which the several inhabitants display.

Why does the author give a definition of instinct at the beginning of the article

A．To ensure the reader knows the meaning.
B．To give the meaning he has chosen to believe.
C．To challenge a commonly held belief.
D．To introduce the topic.