three existing families with special modifications, respectively, for burrowing, climbing, 

 and leaping. Or, to borrow a figure from another hypothetical school, I might remark 

 that the orders, or other natural groups, of placental Mammalia represented by members 

 of the marsupial series have been indicated by different zoologists*, and with no material 

 divergence of opinion ; but not until now has evidence been received of a Marsupial 

 representative of the proboscidian group. 



In certain Mastodons there is a pair of incisive tusks below as well as above : the 

 proboscidians maximize the rodent type as Diprotodon does the Marsupial one. The 

 brain of the Elephant is essentially " lissencephalous," inasmuch as the cerebrum does 

 not extend upon the cerebellum. This position of one primary mass of brain in front 

 of the other is, as stated in the definitions of the subclasses of Mammalia in the cere- 

 bral system f, a more constant and important character than convolution of surface, 

 which in both Ly- and Liss-encephala relates to the bulk of the species and of the brain. 

 So, on the other hand, if a smooth cerebrum overlaps the cerebellum as in the Marmoset, 

 it is essentially " gyrenccphalous." The terms suggested by the superficial character 

 which prevails, with exceptions, are arbitrary, but the most convenient for expressing 

 the more constant characteristics of the brain therewith associated. 



In the extinction of Diprotodon, as of Megatherium, there seems to be an additional 

 exemplification of the fruitful and instructive principle which, under the phrases " contest 

 for existence," or " battle of life," embodies the several circumstances, such as seasonal 

 extremes, generative power, introduction of enemies, &c, under the influence of which 

 a large and conspicuous quadruped is starved out, or falls a prey, while the smaller ones 

 migrate, multiply, conceal themselves, and escape. 



We infer from the fact of remains of young and inexperienced Diprotodons occurring 

 in Australian Caverns with those of Thylacoleo, that the large Marsupial herbivore had 

 its enemy in, and occasionally fell a victim to, the large Marsupial Carnivore ; as at the 

 present day the Kangaroo is laid in wait for by the Thylacyne, or ' Native Wolf, and 

 by the Dasyure, or ' Native Cat.' 



We may speculate upon the possible relation of the first introduction of the Human 

 kind into Australia, and of the subsequent insulation of that land from the rest of the 

 Papuan Continent, to the final extinction in the so restricted territory of all the charac- 

 teristic Mammals which happened to surpass in bulk the still existing, swift-retreating, 

 saltatorial and nocturnal Kangaroos. 



* " On dirait, en un mot, que les marsupiaux forment une classe distincte, parallele a celle des quadrupedes 

 ordinaires et divisible en ordres semblables ; en sorte que si on plac_ait ces deux classes sur deux colonncs, les 

 sarigues, dasyures et peranieles scraient, vis-a-vis des carnassiers insectivores a longues canines, tels que les 

 ' tenrecs et les taupes ; les phalangers et les potoroos, vis-a-vis des herissons et des musaraignes ; les kangaroos 

 proprement dits, ne se laisseraicnt guere comparer a rien ; mais les phascolomes devraient aller vis-a-vis des 

 rongeurs." — Cuvter, Regne Animal, ed. 1817, torn. i. p. 171. " Lea ornithorhyncb.es et les echidnes y formeraient 

 un groupe parallele a celui des odente's." — Op, cit. ed. 1829, torn. i. p. 174. 



t Owe> t , " On the Characters &c. of the class Mammalia," Proceedings of the Linnean Society, 1857. 



