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The classification given by Henri Milne-Edwards (Cours Élementaire d’Histoire Naturelle, Paris, 1855) is as follows:—

(The orders of the classes which follow are not given in the work quoted.)

In England T. H. Huxley adopted in his lectures (1869) a classification which was in many respects similar to both of the foregoing, but embodied improvements of his own. It is as follows:—

We now arrive at the period when the doctrine of organic evolution was established by Darwin, and when naturalists, being convinced by him as they had not been by the transmutationists of fifty years’ earlier date, were compelled to take an entirely new view of the significance of all attempts at framing a “natural” classification.

Many zoologists—prominent among them in Great Britain being Huxley—had been repelled by the airy fancies and assumptions of the “philosophical” morphologists. The efforts of the best minds in zoology had been directed for thirty years or more to ascertaining with increased accuracy and minuteness the microscopic and gross, of all possible forms of animals, and not only of the adult structure but of the steps of development of that structure in the growth of each kind of organism from the egg to maturity. Putting aside fantastic theories, these observers endeavoured to give in their classifications a strictly objective representation of the facts of animal structure and of the structural relationships of animals to one another capable of demonstration. The groups within groups adopted for this purpose were necessarily wanting in symmetry: the whole system presented a strangely irregular character. From time to time efforts were made by those who believed that the Creator must have followed a symmetrical system in his production of animals to force one or other artificial, neatly balanced scheme of classification, upon the zoological world. .The last of these was that of Louis Agassiz (1807–1873), who, whilst surveying all previous classifications, propounded a scheme of his own (Essay on Classification, 1859), in which, as well as in the criticisms he applies to other systems, the leading notion is that sub-kingdoms, classes, orders and families have a real existence, and that it is possible to ascertain and distinguish characters which are of class value, others which are only of ordinal value, and so on, so that the classes of one sub-kingdom should on paper, and in nature actually do, correspond in relative value to those of another sub-kingdom, and the orders of any one class similarly should be so taken as to be of equal