_Genera_, by the details of the execution in special parts. _Species_, by the relations of individuals to one another and to the world in which they live, as well as by the proportions of their parts, their ornamentation, etc. "And yet there are other natural divisions which must be acknowledged in a natural zoölogical system; but these are not to be traced so uniformly in all classes as the former,--they are, in reality, only limitations of the other kinds of divisions." This chapter must be studied in the original text, the arguments by which its conclusions are supported hardly admitting of brief analysis. The most superficial reader will be interested in Mr. Agassiz's account of the mode in which he sought for the natural boundaries of the various divisions, by observing the special point of view in which various eminent naturalists have considered their subject; as, for instance, Audubon, among the biographers of species,-- Latreille, among the students of genera,--and Cuvier, at the head of those who have contemplated the higher groups, such as classes and types. The most indifferent reader will be arrested by the opinions boldly promulgated with reference to species. "The evidence that all animals have originated in large numbers is growing so strong, that the idea that every species existed in the beginning in single pairs may be said to be given up almost entirely by naturalists." "If we are led to admit as the beginning of each species the simultaneous origin of a large number of individuals, if the same species may originate at the same time in different localities, these first representatives of each species, at least, were not connected by sexual derivation; and as this applies equally to any first pair, this fancied test criterion of specific identity must at all events be given up, and with it goes also the pretended real existence of the species, in contradistinction from the mode of existence of genera, families, orders, classes and types; for what really exists are individuals, not species." (pp. 166-167.) Chapter Third is headed, "Notice of the principal systems of Zoology." It is divided into the six following sections: General remarks upon modern systems; Early attempts to classify animals; Period of Linnaeus; Period of Cuvier, and Anatomical systems; Physiophilosophical systems; Embryological systems.