376 Dr. W. Kiikenthal on the Origin and 



characters that we must reg-ard them as descendants of the 

 most primitive mammals. Now, on the basis of our conside- 

 rations on the dentition, we determined that the Multitubercu- 

 lata were the most primitive Mammalia ; the Monotremata 

 therefore must be the descendants of the old Multituberculata. 

 This supposition recently received contirmation owing to the 

 discovery that while the adults of both forms, Platypus and 

 Echidna^ are toothless, the young of the former possess two 

 molars hidden beneath the flesh of the gum, which exhibit a 

 distinctly multitubercular structure. The Monotremata there- 

 fore appear to be really a specialized lateral branch of the 

 Multituberculata. 



The representatives of the second subclass, the marsupials, 

 branched off at a very early period from this ancient stem ; 

 their type of dentition is traceable to a modification of the 

 multituberculate type. Their bodily structure exhibits in 

 general a development occupying a position between Mono- 

 tremata and Placentalia ; and we regard them as an inter- 

 mediate mammalian stage from which the placental mammals 

 have been developed. According to many authors the several 

 orders of placental mammals have sprung from the corre- 

 sponding orders of marsupials, and the former are therefore 

 polyphyletic in origin ; according to others the subclass of 

 the ]?lacentalia originated from a more generalized marsupial 



Let us now examine the evidence, which in any way goes 

 to show that the placental mammals are to be derived from 

 the marsupials. In the first place there are adduced general 

 resemblances and the different degrees of development of the 

 several organs. These arguments we can at once reject as 

 untenable, for the different degree of the resemblance of the 

 organs with those of the two other subclasses may be also 

 explained, if we trace the placental mammals not to the 

 marsupials, but directly to the monotremes. The resemblances 

 would then be simply phenomena of convergence, arising in 

 consequence of adaptation to a similar mode of life. 



A more cogent argument for regarding the marsupials as 

 the ancestors of the placental mammals would be ttie disco- 

 very of specific marsupial characters in the development of 

 individuals belonging to the latter. Such a discovery is 

 supposed to have been made in the finding of remains of the 

 marsupial bones, which in the marsupials serve for the 

 support of the pouch and are quite characteristic structures. 

 Now, however, Wiedersheim *, the latest author on this 



* Wiedersheim, " Die Phylogenie der Beutelknoclien. Eine entwick- 

 lungsgescbichtlicli-vergleicliend anatomische Studie," Zeitscbrift fiir 

 ■wisseuscliaftliche Zoologie, liii., Suppl., 1892. 



