CAUSES NATURAL AND ACQUIRED.
Before proceeding further with the perusal of this third book, the reader will do well to turn back to the two previous books and to compare the one with the other. It will then be seen that a great many of the diseases which are included in the second book, and which, as is there stated, are induced or acquired by those who suffer from them, are closely allied in character to, if they are not identical with, many of the affections which spring from pure natural causes, and take, from their similarities, the same names. The difference, indeed, between the two depends, in most cases, on the circumstance, that whereas in the natural forms of disease the cause is undiscovered or obscure, in the acquired dis ease the cause is entirely known.
As our learning on the question of natural causes advances, all the facts pertaining to self-induced or acquired causes become of great value in explanation; for as the same phenomena must, al most of necessity, be dependent upon what is practically the same cause, the study of the effects of known causes passes into experi mental observation from experiments accidentally and repeatedly presented, when the mind is directed to the study of original com parison and research.