598 SCIENCE PROGRESS 



placental world are not, of course, perfectly continuous with 

 each other, North America being severed by narrow seas, both 

 west and east, from Eurasia. But within very recent geological 

 times, in the Pliocene, and probably during parts of the Pleisto- 

 cene, the continents were so continuous ; there was a bridge 

 across what is now the Bering Strait, and probably another 

 through Greenland, Iceland, and the Faroes. The connection 

 between Africa and Eurasia was also more extensive. Moreover, 

 during the epochs mentioned the climate in the latitudes of 

 the northern bridges was less inhospitable than it now is. It is 

 this recent union which accounts for the wide uniformity of 

 fauna to which I have referred. But, apart from this existing 

 unity, which though very real is superficial and in a sense mis- 

 leading, there is another much more profound unity between 

 the five continents. If, fifty thousand years ago, a land-bridge 

 had arisen between Indonesia and Australia, the mammals 

 of the Oriental Region would speedily have annexed Australia 

 to the placental world. But, as between the other five con- 

 tinents, nothing comparable to such an annexation has ever 

 occurred. The Cainozoic Era, usually called the Age of Mam- 

 mals, would be more accurately styled the Age of Placental 

 Mammals. And, throughout the Cainozoic Era, all the five 

 continents have been populated by placentals. All of them are 

 true placental territory ; none is an annexation. For, although 

 the existing unity is a thing of yesterday, in the beginning of 

 Cainozoic time there were routes by which quadrupeds could 

 wander everywhere, except to Australasia, and each of the five 

 continents received a population of lowly archaic placental 

 mammals. Great geographical separations occurred later ; 

 but, owing to this original union, the five continents have 

 possessed a fundamental unity throughout the present geologi- 

 cal era, and have stood out in sharp contrast to Australia. 



We cannot indicate with any exactitude the geographical 

 positions of these original continental connections. Indeed, 

 in some cases it is far from easy to guess where they were situ- 

 ated. In particular, Africa presents a puzzle ; because in 

 the Eocene period the Mediterranean was evidently much more 

 extensive than it is now. It is, moreover, desirable to beware 

 of creating enormous imaginary trans-oceanic land-bridges. 

 The a priori objections to great geographical changes which have 

 been advanced by Matthew and others are not indeed very 

 convincing. Large areas of land have come up from beneath the 

 sea at different epochs, and hence there is nothing improbable 

 in the idea that considerable masses have sunk. The question 

 is rather one that should be judged on the evidence in each 

 particular case. But we must remember that, when viewed 

 in true geological perspective, the entire Cainozoic Era is very 



