MIGRATION OF TERRESTRIAL VERTEBRATES 307 



have been effected by the intermediary of a bridge- 

 forming Continent, the Antarctic, now in great part 

 subsided under the southern seas. 



We come to the higher or Placental Mammals, 

 whose origin and centre of dispersion are still at 

 this moment an utterly insoluble enigma. All the 

 hypotheses possible have been examined by palae- 

 ontologists. Central Asia, Africa, the Arctic regions 

 (Matthew), Patagonia (Ameghino), even the sub- 

 merged Pacific Continent ! (Haug), have been suc- 

 cessively pointed out as the starting-point of their 

 migrations. It is curious to remark that all these 

 hypotheses, save that of Ameghino, refer to regions 

 palaeontological documents from which are utterly 

 wanting, and this is no doubt why they have been 

 preferred by the authors of these hypotheses. 



Leaving on one side these vain and unproved 

 speculations, we will take the problem in hand by 

 the light of the known facts alone. One great fact 

 to be put in evidence is that we are cognizant of 

 no transition, of no intermediate between the 

 lower Mammals without placentas and the higher 

 Mammals with complete intra-uterine development. 

 A possible common origin of these two great groups 

 is, therefore, entirely hypothetical. We must con- 

 fine ourselves, following the method we have already 

 applied to the Marsupials, to the endeavour to 

 ascertain on what point of the globe, and at what 

 geological date, we see the earliest types of the 

 Placentals make their first appearance. 



These types show themselves almost simultane- 

 ously in three distinct regions : i.e. the United 

 States, France, and Patagonia. In North America, 



