128 



THE INSECT-EATERS. 



placental Insectivora, that several genera of 

 this latter group were regarded as marsupials 

 as long as their organs of reproduction re- 

 mained unknown to naturalists. 



Since the dentition is a remarkably con- 

 servative element in the structure of mam- 

 mals, it may be inferred from all these cir- 

 cumstances that our present-day insectivores 

 are the descendants of older marsupial stocks, 

 which have been continued uninterruptedly 

 through the various geological periods, but 

 in which the reproductive organs have under- 

 gone progressive modifications, while the 

 other organs have remained more or less 

 backward in development. 



The fossil remains of Insectivora are rare, 

 which is easily explained by the small size 

 and the rapid weathering of the bones. Un- 

 doubted remains of Insectivora have, never- 

 theless, been found in the Eocene of Wyoming 

 (North America), and in the phosphorites of 

 Quercy in France belonging to the upper 

 Eocene, remains, however, which cannot be 

 ranked without some violence in any of the 

 families now living. It is not till we reach 

 the Miocene that the remains exhibit the 

 characters of the families of the present day 

 hedgehogs, moles, and shrews. 



Now we understand partially the very 

 singular geographical distribution above in- 

 dicated. In Australia mammals have not 

 developed beyond the primitive marsupial 

 type. Insect-eating marsupials but no true 

 insectivores are to be found there. A 

 singular phenomenon is met with in South 

 America. There we find a number of 

 placental mammals, but the insect -eating 

 marsupials have perpetuated themselves as 

 opossums without further development in 

 this formerly isolated continent. 



Madagascar, separated from the mainland 

 as it has been since the Eocene period, and 

 the Antilles, which have not been connected 

 with the mainland since the Miocene, have 



their own insectivores, derived from peculiar 

 marsupial stocks. North America has only 

 the shrews in common with the Old World, 

 and these perhaps have immigrated from the 

 north, while all the other types of this con- 

 tinent appear to have developed from inde- 

 pendent stocks. Analogous facts are observed 

 in Africa, the East Indies, and the temperate 

 zone of the great Eurasian continent. The 

 separation of closely allied species by wide 

 intervals of land is perhaps explicable on the 

 supposition that allied forms have died out in 

 the intervening tracts. Of the wuychuchol 

 it is at least definitely known that it existed 

 in England in the Quaternary period, so 

 that accordingly the genus Myogale was at 

 that time probably spread over the whole of 

 Europe. 



The rarity and inadequacy of the fossil 

 remains certainly leave great gaps in the 

 derivation of the separate types now living. 

 Of many of them we are still unacquainted 

 with the intermediate forms. 



From the facts that have been already 

 adduced it has been established with perfect 

 clearness that the Insectivora cannot have 

 originated from any one stock, but that separ- 

 ate stocks already existed in Triassic times. 



The bats are flying insectivores, but we 

 cannot trace their ancestors further back than 

 the Eocene, when they already appear with 

 their characteristic wings. It is possible that 

 the ouistitis among the American monkeys, 

 and certain genera of lemurs with an in- 

 sectivorous dentition, are descendants of 

 marsupials, which, under the influence of 

 special conditions of life, have developed to 

 that point at which they have come to 

 approach those lemurs which seem to be 

 descended from ancient hoofed animals (Un- 

 gulata). But we know nothing definite with 

 regard to this, and it is better to recognize 

 the gaps in our knowledge, than to fill them 

 up with more or less arbitrary conjectures. 



