GENESIS OF MAN. 55 



Again, the allantdis is never transformed into a placenta, the corpus 

 callosum is not developed, and there exists a pair of rudimentary 

 marsupial bones. This last character affords almost conclusive 

 proof of the descent of the marsupials from the monotremes. 



The Marsupialia must therefore be regarded as the next group 

 of animals in the regular line of descent which terminates in man, 

 and as forming the seventeenth stage in the development of the 

 human race. Here the cloaca is divided by a horizontal partition 

 into two distinct orifices, both opening externally ; nipples are 

 formed on the mammae, to which the young attach themselves, 

 and the clavicles are distinct from the sternum. In these respects, 

 the marsupials agree with all the higher mammals. The distin- 

 guishing character in which they differ from them, and that from 

 which the name of the sub-class has been taken, is the existence 

 of a remarkable pouch or sack {inarsupium) on the under side of 

 the female, in which the young are placed at a very early period, 

 and there retained until they are able to take care of themselves. 

 This pouch has been aptly likened to a second or supplementary 

 uterus, and the marsupials have accordingly been called by some 

 Didelphia. Our well-known opossum {Didelphys opossum) is our 

 only North American representative ; but in Australia, this group 

 of animals constitutes the greater part of the mammalian fauna. 



The absence of a placenta is the only other important particular 

 in which the marsupials differ from the higher mammals. Indeed, 

 the marsupium seems to constitute a sort of substitute for a pla- 

 centa, and the want of the latter may be regarded as the physio- 

 logical cause of the development of the former. The monotremes, 

 however, are without either, and those who know would do well to 

 explain how these animals are able to. dispense with them both. 



The so-called true Mammalia all possess a fully developed pla- 

 centa, and are therefore distinguished from the two groups last 

 mentioned as forming a third sub-class, the Placentalia. This 

 organ is of great importance in the classification of the higher 

 mammals, its mode of attachment furnishing excellent and reliable 

 general characters. In some, for example, the placenta is decidu- 

 ous from the inner wall of the uterus, while in others it is not, and 

 on this distinction is based the primary division of the whole sub- 

 class into the Deciduata and the Indecidua. The latter are the 

 least perfectly organized, and comprise the Edentata, the Cetacea, 



