The present edition of this work, in its general ground-plan, is essentially identical with those which have preceded it; but it has, nevertheless, been subjected to very considerable modifications in detail. The original aim of the work was to give, in as clear and concise a form as possible, the chief parts of Systematic Zoology, and to give only such a selection from these as could be readily made available by ordinary students. In the present edition this aim has been faithfully adhered to, and, it is hoped, has been more thoroughly carried out than on previous occasions.
While in the main preserving its original basis, the entire work has been submitted to careful revision, and large portions of it have been almost entirely rewritten. A very considerable amount of new matter has been added; but an undue increase in the size of the book has been prevented by the expedient of printing portions of the text in small type. Some space has also been gained by the omission of the synoptical tables of the families of the various orders and classes of the animal kingdom, which were inserted in former editions; and the space thus gained has been largely taken by bibliographical lists, indicating for each great division of animals the more important sources to which the student may look for original information.
Much of the increase in the bulk of this edition is further due to the fact that nearly one hundred additional illustrations have been introduced, which, it is believed, will materially assist in the comprehension of the text. In connection with this subject, the author has to record here his thanks to Messrs A. & C. Black for their kindness in permitting him to use several of the engravings (viz., figs. 61, 64, 65, 66, 69, 71, 73, 75, 76,209, 211, and 212) from his articles on Corals and Cuttle-fishes in the ninth edition of the ' Encyclopaedia Britannica.'
The only other point which appears to require notice relates to the classification here adopted. This classification, as in the previous editions of the work, is based essentially upon the views put forth by Professor Huxley in his masterly treatise, entitled ' Lectures on the Elements of Comparative Anatomy,' published in 1864. The reader will find a good many minor changes in this classification, necessitated by the recent progress of Zoological science. Thus, the new groups of the Hydrocorallinae and Helioporidae of Mosely have been duly recognised; the discovery of the tracheal system of Peripatus has enriched the Myriapoda with a new order: the Therio-dontia of Owen have been added to the already numerous groups of the extinct Reptiles; the orders Odontolcae and Odontotormae of Marsh, collectively forming the new sub-class Odontornithes, are accessions to the class of Birds; and through the researches of the last-named distinguished palaeontologist, the domain of Mammalian life has been extended by the establishment of the Tillodontia and Dinocerata.
In the main, however, the author has not thought it necessary to depart from the broad outlines of the systematic arrangement of animals originally adopted by Professor Huxley, to which he finds himself still able to give his hearty adhesion. The student of some of the more recent German, American, and English zoological publications would, however, find himself confronted with a classification of more modern origin, and in many fundamental points essentially different from the one followed here. Thus, to speak only of conspicuous instances, he would find the Sponges placed with the Coelenterata; the Rugose Corals would be side by side with the Jelly-fishes in the class of the Hydrozoa; the Polyzoa and Brachiopoda would be met with in the "Vermes," in the immediate neighbourhood of the Annelides; and in looking for the Tunicates he would either have to direct his search to the group just mentioned, or he might even light upon the object of his quest at the bottom of the Vertebrate sub-kingdom.
That these and other similar changes have not been adopted here demands a few words of explanation. In the first place, the present work is intended principally for the guidance of general students, and the author is of opinion that it would for this reason be improper to introduce into it any schemes of classification which have not been accepted with tolerable unanimity by naturalists in general. Most or all of the above-mentioned innovations, however, though supported by many and distinguished names, are opposed by others of equal eminence. They may ultimately turn out to be based on nature, but, in the meanwhile, they have not received anything like universal acceptance.
In the second place, most of these proposed changes of classification are founded upon a study of the developmental phenomena of animals. Some highly distinguished zoologists hold that embryological characters will ultimately prove to be the true basis of classification; but in this view the author unfortunately is at present hardly prepared to concur. On the contrary, the author finds himself in the position of being unable to believe that any general system of classification can maintain its ground unless it be based upon the morphological characters exhibited by adult animals. He would not be held as denying, or even as depreciating, the importance of embryological studies, but he is unable to believe that the transitory characters of the young animal can have the same general value in classification - for the purposes, at any rate, of ordinary students - as have the characters drawn from the fully developed organism.
In the third place, if the author had here adopted one of the most modern classifications of the animal kingdom, as advocated by those who hold that embryology is the true key to taxonomy, he would have no guarantee that he might not be called upon to fundamentally alter his arrangement within a year or two. For embryologists are not agreed as to the true import of the phenomena of the development of many animals, and some of our highest authorities in this department of investigation deduce diametrically opposite conclusions from their study of the same phenomena.
Lastly, there are cases in which the author has preferred to retain even an antique classification, rather than accept any one of many arrangements which are based upon methods of inquiry, which are of the greatest possible value to the comparative anatomist pure and simple, but which are unavailable for the purposes of those who merely wish to acquire a general but systematic knowledge of Zoology. The class of Birds offers a case in point. In this instance, the author has preferred to retain, with some modifications, an old and only partially natural classification, because the only available substitutes are arrangements which are purely morphological, and which are based upon the observed variations in single structures. Classifications of this kind, though of the utmost use to the genuine comparative anatomist, can never be thoroughly natural, and they are, at any rate, unsuited for any but very advanced students of the science.
In conclusion, the author can only express his regret that the fact that almost the whole of this work had passed through the press before the middle of October last, should have precluded him from in any way availing himself of, or in some cases even from mentioning in the bibliographical lists of references, some highly valuable works of recent appearance, among which the 'Atlantic,' by Sir Wyville Thomson, the 'Morphology of the Skull,' by Professor Parker and Mr Bettany, and Professor Huxley's 'Comparative Anatomy of Invertebrated Animals,' may be specially alluded to.
United College, St Andrews, April 2, 1878.