14 



THE MAMMALIA. 



cases we must be content to recognize the fact of 

 these gaps and uncertainties, while utilizing in our 

 investigations the assured results that have hitherto 

 been obtained. 



We will speak in the first instance only of land 

 mammals. And here we are at the outset struck 

 by the fact that those boundaries, on the one hand, 

 separate territories that are manifestly continuous 

 at the present day, and, on the other hand, unite 

 others now distinctly separate; and that finally 

 among these last cases we meet with examples in 

 which the mammalian distribution in different 

 regions is strikingly diverse in spite of the apparent 

 correspondence in geographical relations. Such 

 examples are easy to find. 



The channel between the island of Trinidad, the 

 most southerly of the Antilles, and the mainland, 

 at the mouth of the Orinoco, has almost the same 

 breadth as that which separates the British Isles 

 from the European continent. But what a differ- 

 ence is revealed in the mutual relations! The 

 mammalian fauna of England is in no respect 

 different from that of Brittany, while in the case of 

 Brazil and the Antilles almost everything is dif- 

 ferent In Brazil monkeys, carnivores, and eden- 

 tates are uncommonly well represented, and of 

 these groups the Antilles show no trace. Brazil, 

 on the other hand, does not support a single 

 insectivore, while the Solenodons are confined to 

 the Antilles. Only a few rodents, bats, and pec- 

 caries are common to Brazil and the archipelago 

 named. These differences are explained when we 

 learn from geology that at a recent epoch England 

 was continuous with the mainland, that the animals 

 of the Quaternary period, the immediate ancestors 

 of our present mammals, walked dry-footed across 

 the English Channel, while the interval of sea 

 which presents such an insurmountable barrier 

 between Trinidad and Brazil is in process of being 

 filled up by the alluvial deposits of the Orinoco, 

 and formerly was much broader. We might think 

 it strange also that the whole circumference of the 

 Mediterranean is regarded as a continuous sub- 

 region, while the main body of the continent of 

 Africa, apparently directly connected with it, is 

 completely separated by the Atlas Mountains; but 

 for this also geological investigations have furnished 

 the explanation. The Strait of Gibraltar is due 

 to an irruption of the sea of comparatively recent 

 date; more than one isthmus ran from the northern 

 shores of the Mediterranean towards the southern, 

 and the Sahara has constituted, at least in the 



more recent geological epochs, an insurmountable 

 barrier for most species. 



Combining in this way the facts furnished by 

 geology on the one hand, and palaeontology on 

 the other, we can arrive at certain conclusions 

 regarding the origin of the mammals and their 

 geographical distribution, conclusions which must 

 be summed up in the subdivision of the class. Of 

 this we give a tentative sketch. 



Subdivision of the Mammals based on the 

 presumed Order of Evolution. — Passing over in 

 the meantime the aplacental mammals, the mar- 

 supials and monotremes, reserving all that we have 

 to say on this lowest group for the chapters in 

 which we deal with the two orders named, we call 

 to mind by way of preface only these facts: that the 

 oldest mammalian remains are found in the upper 

 Trias; that the only complete jaw known from that 

 epoch, the jaw of the Dromatherium found in North 

 America, is incontestably that of a small insectivore; 

 that the remains derived from the Oolitic strata 

 of Stonesfield and the Purbeck limestones belonging 

 to the upper Jura in England, and from the United 

 States, likewise show in great part an insectivorous 

 dentition; that, however, genera are to be found 

 among them in which the dentition is allied to that 

 of the kangaroo-rats; and that, finally, the last 

 representatives of the marsupials from the Eocene 

 deposits in France exhibit striking affinities to the 

 opossums of America. 



The history of the evolution of the placental 

 mammals, or, in technical language, their phyloge- 

 netic history, begins with the Tertiary formations. 

 With these a number of placental forms suddenly 

 make their appearance, and in the various sub- 

 divisions of the Tertiary epoch, the Eocene, 

 Miocene, and Pliocene, as well as in the sub- 

 sequent Quaternary epoch and the present age, 

 the number of these forms goes on increasing, and 

 the families and tribes still subsisting are gradually 

 developed. 



But it must be remembered that beneath the 

 Tertiary formations there is a vast gap. The rocks 

 lying beneath the Tertiary in the geological scale 

 are the Cretaceous, and from them we know of no 

 mammalian remains whatever. Beneath the Cre- 

 taceous again are the Jurassic and Triassic strata, 

 in which we have indeed mammalian remains, but 

 these solely of aplacental forms. What, then, was 

 the history of the mammals during the intervening 

 period? When did the primitive stocks of the 

 present placental forms first make their appearance.' 



