As might be expected from the great number of species belonging to this order, and their general uniformity of structure, their classification is a matter of some difficulty, and very different views as to their relationships have prevailed at different times. Nowadays, however, zoologists have arrived at something like uniformity of opinion in this matter, and except in some minor points they may be said to be pretty nearly agreed. In the following sketch of the natural history of the Rodents we shall follow the classification proposed by the late Mr. E. R. Alston in the _Proceedings of the Zoological Society_. Mr. Alston accepted the division of the order into two primary groups (sub-orders), proposed fifty years ago by Professor Gervais, and characterised by the number of incisor teeth. The first of these sub-orders, which includes by far the majority of the Rodents, is formed by those species which never at any period of their lives possess more than two incisors in the upper jaw, and have the enamel on these strictly confined to the front surface of the teeth. These are denominated SIMPLICIDENTATA, or SIMPLE-TOOTHED RODENTS. In the second group, which includes only the Hares, Rabbits, and Calling Hares, we have those species which in the adult state possess four incisors in the upper jaw, namely, two large and efficient teeth, and behind these two small, almost rudimentary incisors (_see_ figure of the dentition of the Hare on p. 82). These are called DOUBLE-TOOTHED RODENTS, or DUPLICIDENTATA. SUB-ORDER I.--SIMPLE-TOOTHED RODENTS. Besides the characters derived from the number of incisor teeth above mentioned, several other peculiarities of structure seem to show the existence of a decided difference between the Simple-toothed and Double-toothed Rodents; but most of these are of a rather abstruse nature, and need not be noticed here, the most important additional distinctive characters of the former being that the bony palate is well developed, and that the fibula does not articulate with the calcaneum, or heel-bone; whereas in the Double-toothed Rodents the palate is reduced to a mere bridge between the portions of the upper jaw in which the teeth are inserted, and the fibula does articulate with the heel-bone. The Simplicidentata include a great number of families, and various attempts have been made to group these under larger heads; but it must be confessed that, owing to the way in which the families approach one another, it is difficult to bring them together in sections capable of being very strictly defined. Certain broad principles of relationship are, however, generally recognised, and Mr. Alston represented these by placing the Simple-toothed families under three great sections, the first indicated by Mr. Waterhouse--the Squirrel-like, Mouse-like, and Porcupine-like Rodents.