84 



NATURAL BISTORT. 



I 



greatly in length, being sometimes longer than the body, sometimes reduced to very small proportions, 

 whilst between these two extremes almost every grade of development may be met with. 



The sternum, or breast-bone, is usually long and narrow. Collar-bones are nearly always present, 

 but in a few forms they become rudimentary, or even disappear altogether. The pelvis is long and 

 narrow. The limbs exhibit a very great variety in their development ; in many, the two pairs are 



nearly equal in length, but in the majority the hind limbs are distinctly 

 longer and more powerful than their fellows, and in some groups they 

 attain a most disproportionate length, and serve almost exclusively as the 

 organs of locomotion. On the other hand, in the great majority of the 

 order, the fore limbs serve in a certain degree as hands, and are used for 

 holding the food to the mouth; and in these the radius and ulna, which are 

 always distinct bones, retain the power of rotation. The corresponding 

 bones in the hind limbs (tibia and fibula) are, on the contrary, firmly 

 anchylosed together in two great groups of the order. The feet have 

 usually five toes, but sometimes this number is reduced to four, or even 

 to three, in the hind feet. These toes are armed with claws, which,, 

 however, in one family, acquire more or less of the appearance of hoofs. 



In point of intelligence the Rodentia do not stand high. The brain 

 is comparatively small, and the cerebral hemispheres show no traces of 

 those convolutions of the surface which are characteristic of most Mammals 

 (see figures). The Capybara alone is known to have a few convolutions. 



BRAIN OF BEAVER (from above). * . /, . ". , , ,, , . , m, ,, 



The cerebellum is entirely uncovered by the hemispheres. Ihe organs of 



the senses are generally well developed, and the eyes and external ears, especially, are often of large 

 size. In the Mole Rats and some other burrowing forms, however, the external ears are entirely 

 wanting, and the eyes are very much reduced in size, and in some instances even concealed beneath the 

 skin. The intestinal canal is long, and in all but one family furnished with a distinct caecum. 



The body in the Rodents is generally plump and short, and the head is borne upon a short neck. 

 The limbs also are usually short, so that the belly is close to the ground ; but in some cases all four 

 legs are of moderate length, or, as already stated, the hind legs are enormously developed, forming 

 powerful leaping organs. In general structure, as to a certain extent in habits, there is, in fact, a most 

 striking parallelism between the Rodentia and the Insectivora (see Vol. I., p. 343) ; in both we find 

 arboreal and terrestrial forms, and among the latter some specially organised for burrowing in the earth, 

 and others equally adapted for springing lightly over its surface ; a few, also, in both orders, are 

 aquatic. But here the parallel ceases. The dentition in 

 the two groups is widely divergent, and, as might be 

 anticipated from this circumstance, the food is very dif- 

 ferent ; for, although some Rodents, such as the common 

 Mouse and Rat, are omnivorous, there is no doubt that, 

 as a whole, the Rodents must be regarded as vegetarians. 

 Grass and the leaves of plants and trees furnish some of 

 them with nourishment ; whilst others feed upon fruits, 

 seeds, and nuts, in the consumption of which last the 

 powerful incisor teeth come into play. Many species lay 

 up stores of food for the winter season, of which they 

 pass more or less in a state of torpidity ; and some of these are provided with cheek-pouches, often 

 of considerable size, in which to convey their harvest into their store-houses. 



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), 



BRAIN OF BEAVER. (Profile.) 



