GENERAL CHARACTERISTICS. 



143 



We do not mean to discuss here the 

 characters drawn from the skeleton and the 

 nature of the internal organs, but confine 

 ourselves to the following remarks. The 

 limbs present considerable differences accor- 

 ding to the uses to which they are adapted. 

 Those rodents which use their fore-feet as 

 hands to hold their food while they gnaw, as 

 well as those which climb, swim, and burrow, 

 always have a collar-bone, while this bone is 

 rudimentary or altogether absent in those 

 which use their limbs only for running. The 

 toes are almost always free, seldom connected 

 by a web for swimming. In most cases they 

 are furnished with more or less sharp claws. 

 But there is a South American family, to 

 which our guinea-pig also belongs, provided 

 with true hoofs, whereby it is proved that the 

 distinction between Ungulata and Unguicu- 

 lata, to which so much consequence has often 

 been attached, is after all of doubtful value. 



Lastly, we direct attention to a singular 

 fact first observed in guinea-pigs, but also 

 verified in rats and mice. In all other known 

 mammals, and even in other rodents, as the 

 rabbit, the embryo is formed in such a 

 manner that the central nervous system, 

 which occupies the back, is turned towards 

 the outer part of the egg, the yolk of which 

 is enveloped by the abdominal side of the 

 embryo ; but in the species named the position 

 is exactly the reverse. We now know the 

 cause of this inversion, which at first appeared 

 an inexplicable anomaly; but, nevertheless, 

 when we observe this phenomenon occurring 

 in species separated by the whole breadth 

 of the ocean, we cannot cease to regard it as 

 striking. 



Altogether the rodents form a well-defined 

 order, constituting, as we shall see, one of 

 the oldest types of placental mammals. It 

 cannot be denied that it presents certain 

 affinities to the insectivores and even to the 

 marsupials, and it is, moreover, clear that the 

 dentition of the aye-aye, on the one hand, 

 and the.hyrax on the other hand, indicates 



very well the manner in which the peculiar 

 dental structure of the rodents has been 

 brought about by the loss of the lateral 

 incisors, the canines, and some of the pre- 

 molars; yet it must also be granted that these 

 modifications of the dentition are very old, 

 and that the affinities that may have existed, 

 either with other placental mammals or with 

 the still older marsupials, are very obscure 

 and difficult to demonstrate. 



It is likewise impossible to say anything 

 general concerning the habits and mode of 

 life of the members of this order. They 

 have, indeed, become adapted to all modes 

 of life, to all the conditions which all the 

 different parts of the globe, with all their 

 varieties of climate, present. The Torrid 

 and the Frigid Zones, mountains and plains, 

 withered steppes and soft marshes, are in- 

 habited by them. Wherever vegetable or 

 animal life of any kind is found at all, rodents 

 of some kind are to be met with; in the 

 water and under the ground as well as on 

 the surface. Everywhere we find them 

 exposed to a violent struggle for existence, 

 pursued and preyed upon by carnivorous 

 animals of all classes — mammals, birds, rep- 

 tiles, and even fish, and from these struggles 

 we always see them come forth as victors, 

 not through bodily strength or cunning, but 

 through their incredible fertility. Only a 

 few of them are provided with means of 

 defence, for example, the porcupines, and 

 these bring forth but few young, and have a 

 long period of gestation. But in the case of 

 the great majority of rodents, and especially 

 the small species, the females bring forth a 

 considerable number of young ones, which 

 complete their development in a comparatively 

 short space of time, and soon become capable 

 of reproducing their kind. In this manner 

 the rodents, if the conditions are otherwise 

 favourable, multiply with extraordinary ra- 

 pidity, and in this fact we find the explanation 

 of their frequently sudden appearance in 

 innumerable swarms, which, like swarms of 



