GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



127 



1 he hedgehogs are entirely absent from 

 the mainland of America as well as from the 

 greater part of Africa. In Africa they are 

 found only in the extreme south and in the 

 regions bordering on the Mediterranean, 

 which possess so many other animals in 

 common with Europe. On the other hand, 

 this group inhabits the whole of Europe and 

 Asia as far as the temperate parts of the 

 Western Himalayas. 



Africa has no true moles, these being con- 

 fined to the north temperate zones of the two 

 hemispheres, and advancing in Asia only as 

 far as the Himalayas. The American genera 

 (Condylura, Scalops) are distinguished by so 

 important peculiarities, that many naturalists 

 have separated them from the moles proper in 

 order to make them types of special families, 

 in spite of the similarity in the structure of 

 the fore-paws. 



The shrews alone are spread over the 

 whole of the vast area occupied by the 

 insectivores, from the polar regions to the 

 tropics, and from the snow-clad mountains to 

 the plains. They accordingly present strange 

 developments in order to adapt them to all 

 the conditions of life which our globe has to 

 offer. In face of this wide distribution in all 

 directions the fact already mentioned of the 

 exclusion of the insectivores from South 

 America and from Australia acquires increased 

 importance. 



Our surprise is still further heightened, 

 when we see certain nearly allied species be- 

 longing to one and the same genus separated 

 by wide intervals of land. Of the two species 

 of the true musk-shrews one frequents, as we 

 have seen, the torrents of the Pyrenees and 

 Northern Spain, the other the rivers of the 

 Russian steppes. The two species of Macros- 

 celides, or jumping-shrews, one of which is a 

 native of the Cape, and the other of North 

 Africa, are separated by the greater part of 

 the huge extent of the African continent; 

 and the allied genus Petrodromus likewise 

 inhabits an isolated district in Mozambique. 



All these striking facts we may, not perhaps 

 exactly explain, but at least partially elucidate 

 by a consideration of the origin or descent of 

 the Insectivora. Our Insectivora are the 

 descendants of very old stocks, which can be 

 traced back to the oldest strata in which have 

 been found any remains of mammals at all. 



It is, in fact, the upper strata of the Keuper 

 (belonging to the Triassic system) in Europe 

 and North America, in which have been 

 found jaws and teeth of the oldest mammals 

 yet known. These remains, which have 

 been ascribed to genera called respectively 

 Dromotherium and Microlestes, exhibit a 

 dentition like that of the Insectivora. Are 

 they the remains of genuine Insectivora, of 

 insect-eating marsupials, or, as a distinguished 

 American naturalist, Mr. Marsh, maintains, 

 of the predecessors of the marsupials? 



It is difficult to decide. The large number 

 of the teeth in the single lower jaw that is 

 known, and the distance of the teeth from 

 each other, are in favour of the marsupial char- 

 acter of the remains ; but the absence of the 

 inflexed angle of the hinder part of the lower 

 jaw, which is characteristic in the Marsupialia, 

 induces us to decide in favour of Mr. Marsh. 

 Whichever view one adheres to, so much at 

 least is certain, that the insectivorous type of 

 dentition in these oldest mammals is some- 

 thing that cannot be questioned, and this 

 structure is also seen in some other jaws that 

 have been found in the Purbeck limestones 

 of the upper Jura, remains from which Owen 

 has formed the genera Spalacotherium, Tri- 

 conodon, and others, which he at first assigned 

 to the insect-eaters, afterwards to the mar- 

 supials. 



Now it has not escaped the notice of any- 

 one that these fossil remains, as well as 

 several other similar remains which have 

 been found in the Stonesfield slates belonging 

 to the lower Oolite formation, present the 

 greatest analogy to certain insect -eating 

 marsupials of the present day, which again 

 show so much resemblance to the present 



