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Show 1873.] MR. ST. G. MIVART ON THE LEMURS. 505 principles upon which zoological classification should repose, and the value to be assigned to the various kinds of anatomical resemblance. Until a recent date, zoological classifications reposed on similarities of form and structure accepted simply and without reference to genealogical considerations. Of late years, however, the theory of evolution (and especially the Darwinian form of it) has complicated the inquiry by introducing the distinction between characters which may be reasonably considered to be due to inheritance and others, called adaptive, which m a y be supposed to have originated in necessary conformity to the conditions of life. The doctrine has now been widely received that zoological classification should represent (as far as possible) the genealogical tree of animal life, and therefore that it should repose, by preference, ou characters having a genetic significance, while adaptive characters should be, as much as possible, eliminated. Four questions then naturally suggest themselves :- 1. Is it possible always, generally, or ever, to decide with certainty, of any given set of characters, that some such characters are genetic and certain others adaptive ? 2. Is it possible now to class animals by genetic characters only? and is no zoological classification to be considered satisfactory until based upon such characters ? 3. Is it desirable that animals should not be grouped together into an order unless it can be supposed that they have all sprung from a common ancestor, which was not also the ancestor of any other group (of more or less similar size) belonging to the same class ? 4. Is it desirable that no group of animals which can be reasonably supposed so to have sprung, should be divided into two or more orders ? As to the first question there seems to be great difficulty in arriving at a satisfactory decision. It is true that the coexistence of a great many common characters, such as, e.g., the course of the carotid arteries in all Marsupials and the more or less aborted condition of certain of the digits of the pes in many Marsupials, seem plainly to be due to community of descent ; but many other structures cannot be due to such a cause, and yet seem to be equally uncaused by the exigencies of life-preservation or reproduction. As examples of these latter I m a y refer to the osseous investment of the temporal fossa in Chelonia, Pelo-bates, and Lophiomys, the compound tooth-structure of Orycteropus and Myliobatis, the coexistence of a certain form of dentition with a saltatory habit in Macropus and Macroscelides, the presence of but eight carpal bones in Troglodytes, Indris, and Lepilemur, and the course of the vertebral artery in Auchenia and Myrmecophaga. Thus characters m a y be due to no visible life-exigency, and yet not genetic, while, on the other hand, characters m a y be thoroughly genetic, and yet of great utility. |