Page:Popular Science Monthly Volume 14.djvu/48

38 parts of the body are brought into relation-with each other, in such a way that a disturbance or change in one part shall bring about in another part the liberation of a certain amount of energy, which shall result in change tending to bring the organism into harmony with the conditions which determined the first change. In many cases the action of the nervous system is accompanied by consciousness, and in the higher animals it has a subjective existence as intelligence and volition.

Such, briefly stated, are the most important characteristics of animals as they are manifested by the higher representatives of the group, and it is hardly necessary to call attention again to the fact that none of them furnishes a basis for the absolute separation of animals and plants, or to point out again that many of them are met with only in the higher animals, while others are not confined to animals, but are shared by some plants. The two groups are related to each other somewhat like two streams which have, their sources in the same watershed, but flow in different directions, and through regions of different characters. It is almost impossible to say whether the springs and marshes among which they rise belong to one stream or the other, and they may be connected with both; but, as we pass from this common source, the characteristics of each stream become more marked, until at last their differences, the result of the different conditions to which they have been exposed, overbalance and sink the resemblances which are due to their common source.

We must not suppose that this fact does away with the idea of the essential diversity of animals and plants, or that the distinction between them is any the less real and natural because they can be traced to a common source, and cannot be absolutely defined. As much confusion of ideas exists with reference to this point, it may not be out of place to give an illustration, drawn from another field, to show that a distinction may be real without being absolute:

A person in charge of a small library would find it easy to arrange his books under a few headings: some being devoted to history or science; others to theology or philosophy; others to fiction, poetry, and so on. In most cases the placing of a book would present no difficulty; but, as the size of the collection increased, works would be met with which, though devoted to history, were in part fictitious, and many works of fiction would be found to be historical. Novels would be met with, the aim of which is the exemplification of some psychological, physiological, or religious truth; and so with all the other departments. Most of the new books could still be arranged under headings as readily as in a smaller collection, but every increase in the size of the library would render the inosculation of the various departments of literature more apparent, and would increase the need of a catalogue with cross-references. If the librarian did not confine his attention to the books in his library, but studied the history of the growth of literature, its embryology and paleontology, he would find