Page:Popular Science Monthly Volume 17.djvu/868

848 Our author does not attempt in this volume to give us a complete exposition of mental science. He has, indeed, to deal with subjective psychology and with objective psychology, but he treats these aspects of mental study in relation to the third great mental division, the organic conditions which are the latest results of scientific investigation. From this point of view, the whole subject assumes a new interest, and becomes far more practical than by the previous partial modes of examination. Dr. Bastian has done all that it was possible to do to bring his topic within the range of popular apprehension. Much of his volume will be read with pleasure and profit by all classes; but much of it also requires study and a mastery of its indispensable technicalities. The work can not be put to its proper and highest use unless the objects of which it treats are to a certain extent made real to the mind of the student. Diagrams, of which there are a great profusion, and finely executed, are helps, but they can not be put in the place of the objects they represent without a sacrifice of the first condition of true scientific knowledge—the bringing of the mind into contact with the real things. But there is happily no serious impediment to this manner of study. The brains and nervous parts of animals are to be had anywhere in abundance and in great variety. It is by no means expected that the reader or student will be able to verify the whole course of illustration in this volume, nor is it at all necessary. But it is necessary that he should become acquainted with the rudiments of the exposition by direct observation, so that he will have clear and precise ideas in relation to its subject matter, such as will conduce to a genuine understanding of the general subject. We commend this work especially to teachers, and venture to affirm that if they will form classes in it, not with a view to the slavish acquisition of its contents, but to master portions of it so that the rest may be read intelligently, it will prove invaluable both as a means and an end of education.

Dr. Bastian's book was written as a contribution to the "International Scientific Series"; but, as the author found it impossible to do justice to the subject within the limits prescribed for those works, his volume has been separately issued in this country.

selection may have operated to produce present organic forms in three different ways. Organisms may have been elaborated in structure with a growth in variety and complexity of the conditions to which they were subjected; they may have remained for a long period without any change, when the conditions have been immobile; or, the conditions having become simpler, they may have lost in structure. In accounting for present forms, naturalists have given little heed to the last of these processes, but have endeavored to explain nearly all cases (except those of the parasites, which are generally recognized as degenerate forms) on the basis of the first two processes. In the discourse before the British Association, which forms this volume, Professor Lankester takes issue with this view, and argues in support of the thesis that degeneration is an important process in organic evolution. He contends that many problems are helped to a solution by this hypothesis, which without it are hopelessly obscure, and that the evidence in its favor is of a high order. His argument is based upon the evidence furnished by the changes through which the egg passes in its development into the young creature. As is well known, the forms through which it passes are those that belonged to its ancestors, and these are reached in the order, there is good reason to believe, in which they were acquired by these ancestors. This "recapitulative development" is often very imperfect, many characteristics are obscured or obliterated, but none appear that did not at some time belong to the creature's progenitors. Where these changes are distinct, then, the pedigree of an organism can be traced by them with certainty. A number of cases of degeneration are cited by Professor Lankester, the two most important being the ship's barnacle and the ascidian phallusia. In the case of the barnacle, the egg gives rise to an actively