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THE MAMMALIA. 



vessels; and though all of them become detached 

 at birth from the mucous membrane, this happens 

 with greater difficulty than in the former case. 

 These forms, with the numerous intermediate 

 varieties, have all been comprehended under the 

 general designation of indeciduate placentas. In 

 other cases the cotyledons are all congregated to- 

 gether at certain parts, become intimately united 

 with the maternal organs into a single organ, and 

 at birth at least a portion of the mucous membrane 

 of the uterus, the so-called decidua, always becomes 

 detached and extruded. Mammals with such a form 

 of placenta have been called Deciduata, and among 

 these, two chief forms have been distinguished, one 

 in which the placenta is shaped like a belt sur- 

 rounding the ovum, the so-called zonary placenta, 

 and one in which it has the form of a cake, the 

 discoidal placenta. But these structures represent 

 only higher stages of development, and in many 

 cases approach one another so nearly that, for 

 example, investigators are not yet agreed whether 

 to ascribe to the Prosimii a bell-shaped deciduate 

 placenta, or simple chorionic villi congregated 

 tt^ether at one pole of the ovum. 



The division of the mammalian orders according 

 to this principle would be somewhat as follows: — 



(i.) Indeciduata. (a) With simple chorionic 

 villi (diffuse placenta). Cetacea, Sirenia, Perisso- 

 dactyla, Pigs, Hippopotamuses, part of the Rumi- 

 nants (Camels and the genus Tragulus), Scaly 

 Ant-eaters (Manis), Prosimii; (i) With branched 

 chorionic villi (scattered cotyledons); the rest of 

 the Ruminants. (2.) Deciduata. (a) With zonary 

 placenta: Carnivora, Pinnipedia (Seals), Proboscidea, 

 Hyrax; (i>) With discoidal placenta: Rodentia, 

 Insectivora, Chiroptera, Simiae, Edentata (with the 

 exception of the scaly ant-eaters). 



In whatever way we look at this arrangement 

 it must always be acknowledged that it involves 

 absurdities. It is impossible to deprive the hyrax 

 and the elephant of their affinities to the other 

 Ungulata, in order to rank them with the Carnivora, 

 or to place the scaly ant-caters at the one end of the 

 series, while the common ant-eaters are found at the 

 other; and the group of the Artiodactyla, or even- 

 toed ungulates, shows us that one type may have 

 retained the primitive structure of the placenta, 

 while another very closly allied type (that of the 

 Perissodactyla, or odd-toed ungulates) has attained 

 to a much higher degree of perfection in this organ. 



Distribution in Space.— ln treating of the indi- 

 vidual orders of the Mammalia we have always 



added a concise survey of their geographical dis- 

 tribution, and have sought to combine the results 

 arrived at with the probable presumptions as to 

 the evolution of the stock. We therefore explain 

 in this general introduction the points of view from 

 which we have regarded this part of the subject. 



Formerly it was attempted to explain the un- 

 deniable discrepancies in the distribution of animals 

 over the earth by the assumption that they were 

 ascribable to the surrounding conditions, and especi- 

 ally the climate, the supplies of food, and in short 

 all those influences which can make themselves felt 

 in particular regions. This supposition undoubtedly 

 rested on actual facts. The influences which the 

 surrounding media have exerted on the struggle for 

 existence cannot be denied. If the animals be- 

 longing to northern regions and lofty snow-clad 

 mountains were seen to become white in winter, 

 there was in that single fact manifest proof of the 

 adaptation of the organism to external conditions. 



But such facts were far from exhausting the 

 problem. There remained too many doubts as to 

 a number of circumstances, particularly in view of 

 what had been effected on this field by man. The 

 horses, cattle, and sheep introduced by him into 

 America and Australia succeed there much better, 

 or at least quite as well as at home. Moreover, 

 these animals revert there to the wild condition, and 

 are not only adapted by their own powers for the 

 struggle for existence, but are even victorious in 

 that struggle over the native animals. How then 

 does it happen that such extraordinarily favourable 

 conditions of life have not brought into existence 

 these animals there as in other lands? South 

 America is just as rich in monkeys as the forests of 

 India and Africa. How then does it happen that 

 the apes and monkeys on both sides of the ocean 

 are so different from one another.' And if monkeys 

 can live in all hot regions of both hemispheres, why 

 do we not find antelopes, elephants, rhinoceroses, 

 cattle, and insectivores, as well upon the one as 

 upon the other.' 



I could multiply these examples endlessly, but 

 they would all confirm the insufficiency of the 

 alleged grounds for the discrepancies in distribution. 



The Darwinian theory of the origin of species 

 could not but introduce other points of view. 

 Since our animals of the present day are direct 

 more or less modified descendants of the extinct 

 ones, the geographical distribution of the present 

 day can manifestly be only the consequence of that 

 of the primitive stocks. These have formed, if I 



