THE EOCENE PERIOD. 225 



the west coast, and two in South Africa. It has been demonstrated 

 recently that the proboscideans originated in Africa, and did not emigrate 

 until about the Middle Tertiary. These and other conGiderations that 

 must here be passed by give some plausibility to the view that the 

 placentals had their early evolution in the dark continent during Me so- 

 zoic times, and emerged thence and overran the other continents at the 

 opening of the Cenozoic era. Some part of this plausibility doubtless 

 lies in our ignorance of what took place in "'darkest Africa" in this 

 era, a plausibility that is not without its dangers. 



All these suggestions rest on a slender basis of evidence and have 

 their chief value in giving interest and suggestiveness to the remark- 

 able facts connected with the disappearance of the great Mesozoic 

 dynasty of reptiles, and the apparition of the placentals. 



The rise of placentals was an assignable agency for the downfall of 

 the reptiles, though it cannot be affirmed to have been the actual cause. 

 The placental habit of bringing forth relatively mature offspring, and of 

 nourishing and protecting them, was in itself an immense advantage to 

 the race. The eggs of the reptiles were wholly passive subjects of prey, 

 and during the immature stages after hatching, the young were proba- 

 bly without any intimate relations to the parent for either nourishment 

 or defense. To this great advantage of the placentals at the beginning 

 of life, were added superior agility, as a rule, and higher brain power. 

 It is not surprising, therefore, that the placental invasion resulted in 

 the clumsy, affectionless, small-brained reptiles being driven either 

 into extinction, or into the sedges and rushes, the swamps and lagoons, 

 the coverts of the jungles, the crevices of the rocks, and the various 

 by-ways which the placentals cared least to frequent, and that they 

 have been kept there to this day. 



In a way not implied above, the angiospermous flora may have 

 been a factor in the placental dispersion through the fact that it is the 

 staple source of food of the mammals. It may have been the dispersion 

 of this flora from its originating tract, until it came into contact 

 with the primitive placentals in their originating tract, that caused the 

 rapid spread and evolution of the latter, on a principle often illustrated 

 in human experience, of which perhaps there is no better example than 

 the recent spread of the Colorado potato-beetle when touched in its 

 native region by the western spread of the potato-plant, through the 

 agency of the chief of placentals. This would shift the importance of 



