38 THE POPULAR SCIENCE MONTHLY. 



ous 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 ani- 

 mals 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 water- 

 shed, but flow in difi'erent directions, and through regions of difi'erent 

 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 diff"erences, the result of the difi'erent 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 distinc- 

 tion ma}- 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 Avhich, 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, be would find 



