GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



215 



A large number of the present-day marsu- 

 pials are hand-fo6ted, that is, have the great 

 toe of the hind-foot (the hallux) capable of 

 being opposed to the other toes, whereby a true 

 hand is formed, at least as regards function. 

 The phalangers, the koala, and above all, the 

 members of the opossum family are distin- 

 guished by such a structure of the hind- feet ; in 

 general, accordingly, those genera and families 

 which have retained the insectivorous type of 

 dentition. It must, of course, be admitted that 

 it is very far from being the case that all insect- 

 eaters exhibit this character; the genus Myr- 

 mecobius with its very pronounced insectivor- 

 ous dentition has only four toes on the hind- 

 feet, but except for the peculiar modification 

 seen in the kangaroos, most marsupials have 

 five free toes on the hind-feet, these toes being 

 provided with curved nails or claws. There 

 are, however, some genera in which the 

 thumb or great toe carries a flat nail. On 

 account of these characters the marsupials 

 were formerly even placed among the Ungui- 

 culata (nailed mammals) beside the rodents 

 or the edentates, and this error is still clung 

 to by some naturalists, who concern them- 

 selves more with toes and nails than with all 

 the other parts of the structure. 



So far, I believe, no fossil remains have 

 been found which could give any exact know- 

 ledge of the structure of the foot of the old 

 marsupials; but from the close relationship 

 existing between the genera Peratherium and 

 Galethylax on the one hand, and the opos- 

 sums on the other hand, we may probably 

 conclude that these Eocene genera were 

 likewise hand-footed, and the insectivorous 

 character of still older genera leads to the 

 presumption that they were at least clawed 

 mammals if not likewise hand-footed. 



In consideration of these facts we come 

 therefore to the conclusion that five-toed feet, 

 and indeed feet with free nailed toes, may, 

 with the highest probability, be regarded as 

 the primitive type of mammalian limbs, and 

 that all other forms of limb have been 

 developed out of this form. If this conclusion 

 is correct, it follows further that five-toed feet 

 constructed on this type indicate no advance, 

 but are an inheritance from long- extinct 

 ancestors. The very probable existence of 

 hands in the Eocene opossums likewise 

 proves to us that the hand is an original 

 structure and not the last result of a continu- 

 ous process of development leading to a 

 higher and higher degree of perfection. The 

 only thing that can be urged with any force 

 against this supposition is the fact that in the 

 marsupials the hand is restricted to the hind- 

 limbs. But we meet with a similar pheno- 

 menon in the prosimians and monkeys, which 

 are pre-eminently the climbers among the 

 placental mammals, these having the hands 

 of the hind-limbs much better developed than 

 those of the fore-limbs. 



Our examination of the limbs thus leads us 

 to the same conclusions as the consideration 

 of the dentition: the seemingly more highly 

 developed forms are the original and inherited 

 ones, the simpler forms, on the contrary, the 

 result of a later adaptation. 



We can thus sum up our investigations 

 regarding the marsupials by the statement 

 that they are the oldest known mammals, and 

 that their present geographical distribution, 

 m connection with that of the oldest represen- 

 tatives known to us, does not show their 

 derivation from a single primitive type, but 

 rather proves that they have sprung from 

 many roots. 



