194 



THE MARSUPIALS. 



sorts of teeth represented, incisors, canines, 

 premolars, and molars, it ought to be borne 

 in mind at the same time, that in the herbi- 

 vorous marsupials we meet with the same in- 

 stances of reduction in the number of teeth 

 and specialization of their structure, which 

 have been shown to take place in the pla- 

 cental mammals. These modifications affect 

 especially the incisors, canines, and premolars, 

 and may lead in the end to the formation of 

 a rodent dentition. With very few exceptions 

 the marsupials have four cheek-teeth both in 

 the upper and lower jaw on each side of the 

 mouth, while the number of premolars may 

 be reduced from three, the original number, 

 to two, or even one. The forms of the teeth 

 vary in a remarkable degree, but with a few 

 rare exceptions the cheek-teeth have double 

 or treble roots, and in this respect are in no 

 way different from the best characterized 

 teeth of the placental mammals. 



There is, however, a fundamental distinc- 

 tion as regards the origin and development 

 of the teeth. According to the discovery of 

 Flower the distinction between the milk and 

 the permanent dentition consists in this, that 

 only a single premolar is exchanged shed 

 and replaced. It does indeed happen, for 

 example, in the case of some kangaroos, 

 that, as in the elephants, the front molars are 

 extruded by a rotatory motion of the hinder 

 ones as they grow, but a regular exchange 

 of the front cheek-teeth belonging to the 

 milk dentition for others belonging to the 

 permanent dentition, such as takes place in 

 the placental mammals, does not occur except 

 as regards the single tooth referred to. This 

 is an important character, which, as we have 

 already had occasion to mention in treating 

 of the Carnivora, has served as a means of 

 distinguishing the extinct hysenodons from 

 the marsupials. 



The feet also undergo various modifications. 

 It cannot be repeated too often that the 

 original form of the foot is that with five dis- 

 tinct toes provided with nails. All forms 



with a smaller number of toes and with hoofs 

 are departures from the primitive type clue to 

 a one-sided development. The divergence 

 of the thumb or great toe, which may proceed 

 so far that this digit may become capable of 

 being opposed to the others, is likewise a 

 primitive formation represented from the first 

 in a very pronounced manner in all mammalian 

 embryos. The result is that most of the 

 marsupials still retain the original form of foot, 

 that is, five toes with nails, and some groups, 

 such as the phalangers and the opossums, 

 possess an opposable great toe on the hind- 

 feet. Reduced feet are found especially in 

 the kangaroos and kangaroo-rats; but in these 

 animals the process of reduction follows 

 another rule than in the placental mammals. 

 In the latter it is the first digit that first dis- 

 appears, then the fifth, then the second, and 

 lastly the fourth, so that at last only the middle 

 digit remains as in the horses, or the middle 

 and fourth as in the ruminants. In the 

 marsupials, on the other hand, the toes get 

 degraded in regular sequence from the interior 

 outwards, from the thumb or great toe to the 

 middle digit, so that the kangaroos come to 

 rest in leaping on the abnormally developed 

 fourth and fifth digits. This is an important 

 distinction with reference to the morphology 

 of the feet. 



The body of the marsupials is always 

 covered with thick, mostly soft, seldom coarse 

 hair. 



All the other characters belonging to the 

 bodily structure vary considerably according 

 to the kind of life to which the animals are 

 adapted. With a single exception the marsu- 

 pials are all terrestrial. An American opossum, 

 the yapock (Cheironectes variegatus), has feet 

 adapted for swimming, and lives like the otter 

 or rather like the water-shrews. All the other 

 marsupials are climbers, runners, jumpers, or 

 burrowers. 



The mental qualities, beyond doubt, stand 

 on a very low level, and do not suffice for the 

 struggle for existence where placental mam- 



