II.] ORIGIN OF MONOTREMES. 6l 



that at this period South-eastern Asia was entirely, or to a great 

 extent, devoid of higher mammals. Nor is this unlikely, seeing 

 that the ungulates and carnivores of the lower Eocene of the 

 northern hemisphere would clearly have required time to spread 

 themselves to the southward. Hence it may be suggested that, 

 towards the close of the Cretaceous epoch there was first a migra- 

 tion towards the south-east of the ancestral marsupials (and 

 monotremes) inhabiting the northern hemisphere during the 

 Secondary epoch ; and that similar migrations of the higher mam- 

 mals took place during Tertiary times. 



When once the ancestral polyprotodont marsupials obtained a 

 footing in New Guinea and Australia, where they have since been 

 isolated from any serious competition with the higher mammals, 

 they flourished and developed to a degree which they could not 

 possibly have attained in any other part of the world under 

 existing conditions. And it is doubtless within this region that 

 the more specialised diprotodont types were evolved. Remark- 

 able as it undoubtedly is, the present state of development of the 

 Australian marsupials is nothing to what it was during the 

 Plistocene period, when there lived the giant kangaroos, pha- 

 langers, wombats, diprotodons, and nototheres already alluded to, 

 by the side of which the largest existing species would appear 

 almost dwarfs. The cause of this universal extinction (for uni- 

 versal it is) of all the larger types of mammalian life throughout 

 the world soon after the appearance of man, is one of those 

 problems which at present is not capable of being satisfactorily 

 solved, as not even a glacial period could have made a clean 

 sweep of the whole globe. It may be added that the evolution of 

 the diprotodont marsupials within the limits of the Australian 

 region, points to the conclusion that the outlying discuses of the 

 Austro-Malayan regions are immigrants from the south-east. 



With regard to the monotremes, it has already been mentioned 

 that there is no record of their past history beyond the limits of 

 the Australian region. It can, however, scarcely be doubted that 

 their ancestors came from the north with the primitive marsupials ; 

 and if, as is not improbable, the Secondary and early Tertiary 

 Multituberculata of the northern hemisphere are an allied type, 

 there can be no doubt whatever as to this having been the case. 



