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N 1758 when Linnæus published the epoch-making tenth edition of his "Systema Naturæ" the science of zoology was in a backward condition, having made but little progress for a long period of time. Some important advances, it is true, had been made by the generation immediately preceding that event. Trembley and Peysonnel had proved the animal nature of Hydra and of corals; Linck and Klein had increased the knowledge of the obscure group of echinoderms; Réaumur had continued the brilliant researches of Swammerdam on insects. The discovery of microscopic animals, also, in the preceding century, had opened up new vistas, into which, however, the scientists of the day saw as yet but dimly. Zoology was still, notwithstanding these things, a very crude descriptive science, in which but few fruitful attempts at comparative or philosophical studies had been made.

The cause of this failure to progress rapidly was not the lack of able and earnest zoologists in the preceding ages or even the absence of new discoveries, but the chaotic condition of the zoological classification and nomenclature, which stood in the way of the recognition of the true relationships of animals. A chaos could not become the basis of a system of philosophy. When thus in 1758 Linnæus introduced his fully developed binomial system and arranged all the animals then known to science according to its rules into classes, orders, genera and species he provided the key which should unlock the mysteries of zoology as a science, and disclosed the wonders it contained.

The essential feature of this system and that which was new at the time was the giving to each species of animals of two names, instead of one, or of several, one of which was the specific name and the other the name of the next higher subdivision in the classification, the genus. The other important features were the precisions of the terminology employed, which enables the author to characterize a species in a few words, and the natural arrangement of the classification in which the position of each species indicates the degree of its genetic relationship to all the others.

It is true that predecessors of Linnæus had anticipated many features of his system. The idea of a species was already well fixed before his time and efforts were made to characterize those then known and