Page:Popular Science Monthly Volume 7.djvu/680

660 which have gradually grown up in my mind. Others, I observe, perhaps nearly all evolutionists, are thinking in much the same direction, but I have not yet seen any distinct presentation of the subject.

The movements of the animal body, you will remember, are divided into two great groups, the voluntary and the involuntary or reflex. But between these extremes there are undoubtedly many intermediate terms connecting them. Thus is it in all our science, and still more in our systematic teaching of science. Our distinctions are far more trenchant than the distinctions in Nature. It must and ought to be so, for we must get firm hold of the types first, and then we are prepared to study the intermediate gradations. Of the intermediate terms in this case there are two which are quite distinct. Including the extremes, therefore, we have four kinds of animal movements:

1. The perfect voluntary movements.—These require the full, constant, and immediate exercise of the will; and, when the movement is complex, requiring in addition the whole thought and attention fixed, often painfully fixed, on the movement. In this category are nearly all movements when accomplished for the first time.

2. Habitual movements.—These are semi-volitional. They are removed from thoughtful attention, from immediate and painful effort of the will. A general superintendence only of the will is necessary. When any thing goes wrong the mind takes cognizance and corrects it by direct act of the will, and the movement falls, for the time being, into the first category; but otherwise the thoughts and attention may be directed to something else. These are, therefore, to some extent, automatic. Such are, in man at least, the movements in walking, flying, swimming, speaking, playing on a musical instrument, etc. These were, in all cases, at first movements of the first kind, but fell into the second category by repetition. They are acquired, therefore, wholly by individual experience.

3. Instinctive movements or acts.—These are still farther removed from the category of the first group. They are removed, not only from thoughtful attention, but also from individual experience. If we compare them with habitual acts, they are inherited habits. They are evidently the result of inherited brain-structure, but they are not yet wholly removed from the sphere of consciousness and will. Such are the actions of bees and other insects already described.

4. Lastly, Reflex movements.—These are wholly automatic. They are wholly removed not only from thoughtful attention and individual experience, but also from consciousness and will. These are therefore the extreme type of movements determined with the greatest precision by inherited structure of the nervous centres. Such are the movements of the heart, the stomach, the intestines, etc.

Now, of these four kinds of acts, 1 and 2 and 3 are evidently formed the one from another, i. e., 2 from 1 and 3 from 2. The fourth I cannot account for in a similar way, for it must have preceded all the