MAMMALS 



and Proviverra ) are primitive Carnivora, which 

 show skeletal affinities with Marsupials and Insec- 

 tivores. Not less remarkably generalised are the 

 Condylarthra (e.g. Phenacodus and Periptychus), 

 primitive Ungulates showing affinities with Artio- 

 dactyles and Perissodactyles, with Hyracoidea and 

 (through the Creodonta) with Carnivores, and 

 (according to Cope) even with the Lemurs. In the 

 same way the palaeontologists find transitions 



Fig. 1. Slab of Rock showing the left lateral aspect of 

 the skeleton of Pkenacodiu primenun; from tlie Lower 

 Eocene of North America : actual size of Blab, 49 inches 

 in length. ( Alter Cope.) 



between Insectivorous, Lenmroid, and Creodont 

 tvpes, between Perissodactyles and ProWseidea 

 ( Oinocerata and Coryphodonts ), between Rodents 

 and Ungulates ( Mesotherium and Toxodon). So, 

 too, a common base has been found for dogs and 

 bear*, for pig< and sheep, for deer and chevrotains ; 

 but it is enough for our purpose to emphasise the 

 fact, which rapidly progressive research continually 

 corroborates, that in early Tertiary times there 

 persisted numerous generalised mammals which 

 united many of the characteristics of our extant 

 orders. 



Distribution in Sjrtce. Referring to the article 

 on GEOGRAPHICAL DISTRIBUTION for the general 

 results reached by the labours of Murray, Wallace, 

 Sclater, and others, we shall content ourselves with 

 a few illustrations showing the importance of the 

 inquiry in regard to mammals. Perhaps the most 

 striking of these concerns the great insular region 

 of Australasia, where, with the exception of some 

 bate and marine mammals which transcend the usual 

 limits, of some rats and mice, and of forms intro- 

 duced bv man, the whole mammalian fauna con- 

 sists of .Marsupials and Monotremes. As all extant 

 Marsupials, with the exception of the American 

 opossums, are now Australasian, and as fossil 

 remains of the sub-class are found as far away 

 as Kurope, we have here one of the most remark- 

 able cases of gradual restriction and of the saving 

 results of feologioal changes. For, whatever the 

 precise details may be, there seems no doubt that 

 geological insulation saved the Marsupial immi- 

 grants to Australia from the jaws of their pursuers. 



In the Lemuroid group, again, we find 'one of 

 the most singular phenomena in geographical <Hs- 

 tri bution.' For out of a total of fifty species 

 thirty are confined lo the one island of Mada- 

 gascar, the remainder occurring through tropical 

 Africa and in restricted portions of India and the 

 .Malay Islands facts from which it is fairly con- 

 cluded that in the insulated Madagascar ' the 

 lowly organised Lemuroids diverged into special ised 

 forms of their own peculiar type, while on the 

 continents they have to a great extent become 

 exterminated, or have maintained their existence 

 in a few cases in islands or in mountain-ranges.' 



The Edentata (sloths and ant-eaters) have also 

 a very restricted distribution in modern times, for, 

 with the exception of the scaly ant-eaters or Manid;i> 



(Ethiopian and oriental in range) and the African 

 aardvark, the home of the order is in South 

 America, where, moreover, in Pliocene times there 

 flourished a giant race ' rivalling in bulk the 

 rhinoceros and hippopotamus.' 



Just as naturally as terrestrial mammals are 

 absent from Oceanic islands, so the aquatic 

 Cetaceans have a world-wide distribution, and 

 the Sirenians almost as wide as= required conditions 

 of temperature will admit. But it must be clearly 

 noted that when we follow in detail the distribu- 

 tion even of bats, whose great powers of flight free 

 them from the limitations imposed on terrestrial 

 mammals, we find that the inhabitants of special 

 regions are usually marked off with perfect de- 

 finiteness. The same local definiteness holds true 

 of the world- wide (Australia always excepted) 

 distribution of Ungulates, Rodents, and Carni- 

 vores, and is signally illustrated, for instance, in 

 the complete absence of Insectivora from South 

 America alone, or in the striking differences 

 between Old and New World monkeys. 



Development. The ova, which are small and 

 poor in yolk except in Monotremes, burst from the 

 ovaries into the upper ends of the oviducts, may be 

 fertilised by ascending spermatozoa, and with the 

 above exception develop in the lower portion of the 

 female duct known as the uterus. In the ovi- 

 parous Monotremes the segmentation is partial, 

 like that of birds and reptiles ; in all the others 

 the egg segments completely. The development 

 proceeds in a fashion somewhat different in detail 

 From that of the other vertebrates, but it is more 

 ini]>ortaiit to notice that in the Placentals a close 

 vascular connection is speedily established between 

 the embryo and the wall of the uterus. In the 

 hedgehog, which is a remarkably central type, this 

 connection is first of all maintained simply by the 

 outermost layer of the developing egg ; but this is 

 soon abetted by a union between the yolk-sac and 

 the maternal wall, which in turn gives place to 

 the true placenta, mainly due to the Allantois 

 (q.v. ). The final result is an interlocking of the 

 maternal tissue with that of the foetal membranes, 

 and the whole life of the embryo depends on the 

 intimacy of this interlocking, by which the blood 

 of the mother is vitally though not directly united 

 with that of the offspring. At birth the union is 

 I severed, and the embryonic part of the placenta, 

 j with more or less of the associated lining of the 

 uterus, is discharged. The form anil structure of 

 the placenta vary considerably in different orders, 

 and have furnished important aid in determining 

 relationship. Of mammals as of other animals it 

 is true that the individual development recapitu- 

 lates, in general outline, the history of the race, 

 for the life logins at the beginning again in a 

 single cell, divides into a ball of cells, acquires a 

 layered body, and passes from stage to stage pre- 

 senting successively the general features of a verte 

 brate, of a reptilian (?), of a simple mammal, of 

 an insectivore, and finally of a young hedgehog. 

 Nursing remains somewhat crude in the oviparous 

 Monotremes, which are destitute of teats, but the 

 embryos have a considerable store of yolk which 

 serves as preliminary capital. The eggs of the 

 duckmole are laid in a nest, those of the Echidna 

 seem to be borne in a temporary pouch suggesting 

 that of Marsupials. In both cases the young lick 

 the bare patch of skin on which the mammary 

 glands open. The non-placental Marsupials are, 

 in a sense, as Professor Flower says, ' the most 

 mammalian of mammals,' since most of them carry 

 their prematurely-born young in an external pouch 

 surrounding the teats, whence the milk is forced into 

 their passive mouths. In the placental mammals 

 the young are born in a more advanced state, 

 though still requiring much care. They are able 



