224 GEOLOGY. 



and what might have originated in the high-latitude area, since both 

 of these would necessarily move southward in invading the continental 

 regions where their relics are chiefly found. 



All hypotheses that postulate an origin in Eurasia or North America 

 are somewhat, though not absolutely, dependent on the hypothesis that 

 the placentals descended from the non-placentals of these regions. Some 

 paleontologists have, however, entertained the view that the placental 

 and non-placental branches diverged from a common stock at an early 

 stage, probably far back in Mesozoic times. This view lends whatever 

 strength it may have to the hypothesis that the placentals arose in 

 some region whose record has not yet been carefully studied, because 

 the transition forms do not appear, at least in sharp definition, in the 

 European or the American Mesozoic record. 



Of such uninvestigated regions, Africa presents the most favor- 

 able antecedents for placental origination. Australia is excluded, 

 because placentals do not seem to have ever lived there, until recently 

 introduced, and South America also, for its placentals seem to be pro- 

 vincial and limited in type-range, as though they were the offspring of 

 a branch that became isolated early and developed by itself. The com- 

 mon parents of all placentals should be rather markedly comprehensive. 

 South America seems also to have been in migratory relations with 

 Australia after the appearance in other lands of the primitive placentals, 

 for carnivorous marsupials of comparatively recent Australian types 

 lived there. In Africa, on the other hand, the placentals are compre- 

 hensive in type-range, are highly developed, and widely deployed, and 

 have a remote mammalian ancestry, with living relics of primitive stock. 

 It will be recalled that in the Permo-Triassic times, when the amphib- 

 ians were deploying into the ancestral branches from which all reptiles, 

 birds, and mammals have probably descended, the Karoo beds of South 

 Africa displayed an extraordinary vertebrate fauna in which the mam- 

 malian strain of reptiles was a conspicuous feature. More definite fore- 

 shadowings of the coming mammalian race were shown there than 

 anywhere else, notwithstanding the relative scantiness of our knowl- 

 edge of the "'dark continent." At present, Africa is almost the sole 

 home of the least modified survivor of one of the great branches of the 

 primitive placentals, the Condylarthra, in the form of the hyrax, the 

 coney of the Bible, which has crept out into Syria, but is otherwise 

 confined to Africa, where one species is found in the northeast, one on 



