RELATIVE VIABILITY IN MAMMALS AND BIRDS. 447 



longevity of Squirrels and other members of the Sciurid?e, and it 

 was a surprise to find that a Squirrel had lived over fifteen years, 

 a Flying-Squii-rel over thii'teen, a Souslik eleven, and a Marmot 

 over thirteen. It first occurred to me that as these animals 

 have not an individuality that is very obvious, mistakes with 

 regard to individuals might have confused the records. Such an 

 explanation, however, does not cover the facts ; it would apply at 

 least equally well to the Common Squirrel, the 77 examples of 

 which showed no case of great longevity and a very low average, 

 and it could not possibly account for the many cases of high 

 longevity, where only two or three examples are recorded through- 

 out the period of thirty- three years. We must take as established 

 the striking fact that squirrel-like animals have a very high 

 potential longevity in proportion to their size and in comparison 

 with other vegetable-feeding forms, as the latter are on the whole 

 relatively short-lived. The average duration and therefore the 

 viability of the group appears to vary much ; some species are 

 almost uniformly unsuccessful, whilst others show a distribution 

 of durations veiy close round the average. Taking the group as 

 a whole, however, it has shown a relatively high viability just as 

 it has a relatively high maximum longevity. It is particularly 

 interesting to reflect that during the greater part of the period to 

 which the figures relate, most of these squirrels were housed in 

 outdoor cages freely open to the air, with no artificial heating, 

 but with small nesting-boxes as retreats. 



rUorrrvT.TT^2c- No. of Av. diar. Max. dur. 



L.ASTORIDJ!:. j^^^j^_ in months, in months. 



Castor fiber (European Beaver) ... 4 4-2 7 



C . canadensis (Qa.n2id\?cii ,, ... 30 40*3 130 



The great difference in viability between European and Canadian 

 Beavers is remax-kable. The average of the latter is relatively 

 low on account of the large proportion that lived only a few 

 months ; if two or three months were survived these beavers usually 

 lived some years. The number of European beavers within the 

 period is small, and it may well be that it included no healthy, 

 nearly adult individual. 



Myoxid^. 



No. of Av. duv. Max. dur. 



Indir. in months, in months. 



iY^/oajMS, whole genus 23 14'6 35 



J/^/o.TtfS cZr^/ffls (Oak Dormouse) 6 12*5 30 



M.glis (Fat „ ) 3 14-3 34 



M. quercinus (Garden ,, ) 14 15*5 35 



Micscardiii'us avellaoiarius (Common 17 3 '5 13 

 Dormouse). 



The relative low viability of, or failure with, the common 

 English Dormouse is remarkable. A high longevity is not to be 

 anticipated for dormice, althoiigh I am unaware of any recorded 



