the mollusca are not only removed from that type by many diversities, but a number of them have such affinities with the Radiate type, that it is only in quite recent days that the whole class of Polyzoa (or Bryozoa, as they are also called) has been removed from the Radiata, and ranged under the Mollusca. To quit this topic, and recur once more to the five divisions, we have only the broad outlines of the picture in Vertebrata, Mollusca, Articulata, Radiata, and Protozoa; but this is a good beginning, and we can now proceed to the further sub-divisions. Each of these five sub-kingdoms is divided into Classes; these again into Orders; these into Families; these into Genera; these into Species; and these finally into Varieties. Thus suppose a dwarf terrier is presented to us with a request that we should indicate its various titles in the scheme of classification: we begin by calling it a vertebrate; we proceed to assign its Class as the mammalian; its Order is obviously that of the carnivora; its Family is that of the fox, wolf, jackal, &c., named _Canidæ_; its Genus is, of course, that of _Canis_; its Species, terrier; its Variety, dwarf-terrier. Inasmuch as all these denominations are the expressions of scientific research, and not at all arbitrary or fanciful, they imply an immense amount of labour and sagacity in their establishment; and when we remember that naturalists have thus classed upwards of half a million of distinct species, it becomes an interesting inquiry,—What has been the guiding principle of this successful labour? on what basis is so large a superstructure raised? This question we shall answer in the next chapter. FOOTNOTES [8] DRAPER: _Human Physiology_, p. 288. [9] LEUWENHOEK: _Select Works_, ii. p. 210. His figures, however, are very incorrect. [10] See LEYDIG: _Ueber den Bau und die systematische Stellung der Räderthiere_, in SIEBOLD _und_ KÖLLIKER’S _Zeitschrift_, vi., and _Ueber Hydatina Senta_, in MÜLLER’S _Archiv_: 1857. [11] POUCHET: _Hétérogénie, ou Traité de la Génération Spontanée_, 1859, p. 453. [12] The _Tardigrade_, or microscopic _Sloth_, belongs to the order of _Arachnida_, and is occasionally found in moss, stagnant ponds, &c. I have only met with four specimens in all my investigations, and they were all found in moss. SPALLANZANI described and figured it (very badly), and M. DOYÈRE has given a fuller description in the _Annales des Sciences_, 2nd series, vols. xiv. xvii. and xviii. [13] DAVAINE in _Annales des Sciences Naturelles_, 1858, x. p. 335. [14] SPALLANZANI: _Tracts on the Natural History of Animals and Vegetables_: Translated by Dalyell, ii. p. 129.