in extent, the careful examination of large suites of specimens revealed an unexpected amount of variability in species; and Darwin’s investigation of the phenomena presented by animals under domestication proved that forms, morphologically as distinct as admitted natural genera, could be produced by selective breeding from a common stock. The only genus of animals of which we possess a satisfactory, though still not quite complete, ancestral history, is the genus Equus, the development of which in the course of the Tertiary epoch from an Anchitherioid ancestor, through the form of Hipparion, appears to admit of no doubt. And all the facts of geology and palæontology not only tend to show that the knowledge of ancestral development is likely long to remain fragmentary, but lead us to doubt whether even such fragments as may be vouchsafed to us by the extension of geological inquiry will ever be sufficiently old, in relation to the whole duration of life on the earth, to give us positive evidence of the nature of the earliest forms of animals. In the case of an existing animal, it is possible to determine its adult structure and its development, and therefore to assign its place relatively to other animals, the structure and development of which are also known; and, in the case of an extinct animal, it is possible to ascertain certain facts of its structure, and sometimes certain facts of its development, which will justify a more or less positive assignment of its place relatively to existing animals. So far, Taxonomy is objective, capable of proof and disproof, and it should leave speculation aside, until speculation has converted itself into demonstration. In the present rapidly shifting condition of our knowledge of the facts of animal structure and development, however, it is no easy matter to group these facts into general propositions which shall express neither more nor less than is contained in the facts; and no one can be more conscious of the manifold imperfections of the following attempt at such a classification than the author of it. In certain of the lower animals, the substance of the body is not differentiated into histogenetic elements; that is, into cells[3] which, by their metamorphoses, give rise to tissues. In all other animals, on the other hand, the protoplasmic mass, which constitutes the primitive body, is converted into a multitude of cells, which become metamorphosed into the tissues of the body. For the first of these divisions the old name of Protozoa may be retained; for the second, the title of Metazoa, recently proposed by Haeckel, may be conveniently employed. The subjoined synopsis indicates the general relations of the different groups of the animal kingdom.