[135] Γένη.
[136] Εἴδη.
These passages afford us sufficient ground for placing Aristotle at the head of those naturalists to whom the first views of the necessity of a zoological system are due. It was, however, very long before any worthy successor appeared, for no additional step was made till modern times. When Natural History again came to be studied in Nature, the business of Classification, as we have seen, forced itself upon men’s attention, and was pursued with interest in animals, as in plants. The steps of its advance were similar in the two cases;—by successive naturalists, various systems of artificial marks were selected with a view to precision and convenience;—and these artificial systems assumed the existence of certain natural groups, and of a natural system to which they gradually tended. But there was this difference between botany and zoology:—the reference to physiological principles, which, as we have remarked, influenced the natural systems of vegetables in a latent and obscure manner, botanists being guided by its light, but hardly aware that they were so, affected the study of systematic zoology more directly and evidently. For men can neither overlook the general physiological features of animals, nor avoid being swayed by them in their judgments of the affinities of different species. Thus the classifications of zoology tended more and more to a union with comparative anatomy, as the science was more and more improved.[137] But comparative anatomy belongs to the subject of the next Book; and anything it may be proper to say respecting its influence upon zoological arrangements, will properly find a place there.
[137] Cuvier, Leç. d’Anat. Comp. vol. i. p. 17.
[418] It will appear, and indeed it hardly requires to be proved, that those steps in systematic zoology which are due to the light thrown upon the subject by physiology, are the result of a long series of labors by various naturalists, and have been, like other advances in science, led to and produced by the general progress of such knowledge. We can hardly expect that the classificatory sciences can undergo any material improvement which is not of this kind. Very recently, however, some authors have attempted to introduce into these sciences certain principles which do not, at first sight, appear as a continuation and extension of the previous researches of comparative anatomists. I speak, in particular, of the doctrines of a Circular Progression in the series of affinity; of a Quinary Division of such circular groups; and of a relation of Analogy between the members of such groups, entirely distinct from the relation of Affinity.
The doctrine of Circular Progression has been propounded principally by Mr. Macleay; although, as he has shown,[138] there are suggestions of the same kind to be found in other writers. So far as this view negatives the doctrine of a mere linear progression in nature, which would place each genus in contact only with the preceding and succeeding ones, and so far as it requires us to attend to more varied and ramified resemblances, there can be no doubt that it is supported by the result of all the attempts to form natural systems. But whether that assemblage of circles of arrangement which is now offered to naturalists, be the true and only way of exhibiting the natural relations of organized bodies, is a much more difficult question, and one which I shall not here attempt to examine; although it will be found, I think, that those analogies of science which we have had to study, would not fail to throw some light upon such an inquiry. The prevalence of an invariable numerical law in the divisions of natural groups, (as the number five is asserted to prevail by Mr. Macleay, the number ten by Fries, and other numbers by other writers), would be a curious fact, if established; but it is easy to see that nothing short of the most consummate knowledge of natural history, joined with extreme clearness of view and calmness of judgment, could enable any one to pronounce on the attempts which have been made to establish such a principle. But the doctrine of a relation of Analogy distinct from Affinity, in the manner which has recently been taught, seems to be obviously at variance with that gradual approximation of the classificatory to the [419] physiological sciences, which has appeared to us to be the general tendency of real knowledge. It seems difficult to understand how a reference to such relations as those which are offered as examples of analogy[139] can be otherwise than a retrograde step in science.
[138] Linn. Trans. vol. xvi. p. 9.
[139] For example, the goatsucker has an affinity with the swallow; but it has an analogy with the bat, because both fly at the same hour of the day, and feed in the same manner.—Swainson, Geography and Classification of Animals, p. 129.
Without, however, now dwelling upon these points, I will treat a little more in detail of one of the branches of Zoology.
[2nd Ed.] [For the more recent progress of Systematic Zoology, see in the Reports of the British Association, in 1834, Mr. L. Jenyns’s Report an the Recent Progress and Present State of Zoology, and in 1844, Mr. Strickland’s Report on the Recent Progress and Present State of Ornithology. In these Reports, the questions of the Circular Arrangement, the Quinary System, and the relation of Analogy and Affinity are discussed.]