[Footnote 2: With certain modifications accounted for in their historical "descent" with modification from a common ancestor. See Scott: _Theory of Evolution_.]
[Footnote 1: The difference between the two is largely one of complexity. By a reflex is meant a very simple and comparatively rigid response; by an instinct a series of reflexes such that when the first is set off, the remainder are set off in a regularly determinate succession.]
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 death, or of self-preservation. He has probably attained to no one of these 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 barking 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.[2]
[Footnote 2: James: _Psychology_, vol. II, p. 384.]
Similarly, the baby's reaching for random objects, and sucking them when seized, its turning its head aside, when it has had enough food, its crying when alone and hungry, are not, for the most part, deliberate methods invented by the infant to maintain its own welfare, but are almost as automatic as the number of sounds omitted by the cuckoo clock at midnight.
Why do men always lie down, when they can, on soft beds rather than on hard floors? Why do they sit round the stove on a cold day? ... Why does the maiden interest the youth so that everything about her seems more important and significant than anything else in the world? Nothing more can be said than that these are human ways, and that every creature _likes_ its own ways, and takes to the following of them as a matter of course.... Not one man in a billion, when taking his dinner, thinks of utility. He eats because the food tastes good, and makes him want more. If you ask him _why_ he should want to eat more of what tastes like that, instead of revering you as a philosopher, he will probably laugh at you for a fool.[1]