RELATIVE VIABILITY IN MAMMALS AND BIRUS. 465 



It is impossible to separate, by any important characters, the 

 Kangaroos, Wallabies, Kock Kangaroos, Rat- Kangaroos, and 

 Phalangers. They form a series roughly corresponding with the 

 ruminants amongst higher mammals, and have similar habits 

 and food. The alimentary tract, although much simpler in 

 structure than that of ruminants, possesses a capacious hind-gut 

 and ctecum, and they are therefore animals that might be 

 expected to have low potential longevity and poor viability. 

 The maximum duration in the group, nearly 14 years in the case 

 of a kangaroo, is more or less what might be expected, and on 

 the whole there is a rough coiTespondence between size and 

 longevity. The average duration is low compared Avith the 

 maximum duration, and it is a fair inference that the viability is 

 not good. Amongst the Phalangers, however, thei^e are remark- 

 able exceptions, the maximum duration and the average duration, 

 in a number of cases, being unusually high in proportion to size. 

 In reflecting upon this I was disposed at first to distrust the 

 records ; in animals so little individualized it might well be that 

 mistakes had been made as to individuals, and hence that wrong 

 durations had been assigned in the records. Although this may 

 well have happened in some of the cases, it is not a sufficient 

 explanation. A period of 33 years is covered by the records, and 

 even where the number of individuals was largest, the number of 

 individuals alive at any one time was probably not more than 3 or 

 4 at most, and the chance of confusion was therefore not great ; 

 there are moreover cases of remarkable duration where the 

 number of individuals was so small as to make confusion prac- 

 tically or absolutely impossible, — of two Yellow-bellied Phalangers, 

 for instance, one lived over 10 years, and one Short-eared 

 Phalanger lived over 7 years. It is clear that these animals have 

 a high viability. In the period covered by the record some of 

 them were kept in fairly roomy cages, but in an artificially heated 

 house without access to the open air. Others were kept in small 

 outdoor cages with no artificial heat. Unfortunately I cannot 

 now trace which individuals lived in these very difierent kinds of 

 quarters. Since the period in question several species have 

 thriven in a shed open to the outer air, without artificial heat, 

 but with protection from wind. Capt. Flower records a Great 

 Grey Kangaroo, a Black-faced Kangaroo, and a Wallaroo, each 

 alive after 10 years' duration. 



Phascolomyid^. J?°-.f ^J-Jl\- .M«^'-<^i"-. 



Inrtiv. Ill months, in months. 



Phascolomi/s, whole genus 13 74 212 



P. latifrons (Hairy -nosed Womhat) 4 88 212 



P. mitchelli (Common „ ) 9 68 189 



A record of nearly 18 years for a Wombat seems to show that 

 these animals have a high potential longevity in joroportion to 

 their size, whilst their viability, as shewn by the average duration, 

 is good. Throughoiit the period, they were kept in an unheated 

 shelter with free access to the open air. 



