MAMMALIA 353 



four on each side; usually but one tooth of the milk set, the fourth 

 premolar, is functional. 



In general it may be said that the marsupials occupy a position 

 intermediate between the monotremes and the placental mammals. 

 They have undergone an elaborate adaptive radiation, occupying in 

 their native countries most of the life zones that are in other parts 

 of the world occupied by the placentals. Their favorite life zone is 

 the arboreal and they seldom invade the aquatic zones. There are 

 some highly specialized cursorial types; some sub-terrestrial, fossorial 

 types; and some semi-volant types. They have produced several 

 giant forms, now extinct, but recent forms are small or of moderate 

 size. 



While they were at one time numerous and fairly well distributed 

 over Europe and North America they are now almost confined to 

 Australasia. A number of species of opossums and the rat-like Cceno- 

 lestes belong to the American continent, mostly to South America. 

 None are found in Europe, Asia or Africa to-day. 



It is believed by some authorities that the marsupials spread to 

 Australia and South America over the hypothetical Antarctic land 

 bridge and were subsequently cut off before the placental mammals 

 were evolved. They have persisted in Australia largely because they 

 have escaped competition with the larger and more capable modern- 

 ized mammals that ruled the other continental bodies. A more care- 

 fully considered theory, however, would derive the marsupials from 

 northern forms that migrated southward to escape the rigors of the 

 early glacial epochs, and reached Australia and South America before 

 the onset of the dominant placentals. The cutting off of Australasia 

 gave them their best opportunity for adaptive expansion. 



It is not now believed that the marsupials represent a stage in 

 the evolution of the placental mammals; rather it is thought that 

 they represent the adaptive radiation of a primitive mammalian stock 

 that arose far back in the Mesozoic (probably during the Jurassic) 

 and has had an evolution of its own, somewhat less successful and 

 slower than that of the modernized groups. They show many evi- 

 dences of racial senility and some of their supposedly primitive fea- 

 tures may well be the products of regressive processes. The fact 

 that there are only traces of the milk dentition, and the occurrence 

 in some species of a transitory allantoic placenta, have been inter- 

 preted as retrograde conditions and as evidences in favor of the idea 



