RELATIVE VIABILITY IN MAMMALS AND BIRDS. 451 



The Agoutis and Cavies are another instance of small Rodents 

 which come from various kinds of climates, but which have been 

 treated as outdoor animals and show a relatively high maximum 

 and average longevity. 



Leporid^. I^-f 



Lepus, whole genus 23 



L. americ«)W(s (American Hare) 3 



L. hrasiliensis (Brazilian ,, ) 3 



L. eiirojjceus (Common „ ) 10 



L. nigricollis (Black-necked,, ) 1 



L. sylvaticvjS (Wood ,, ) 4 



L. variabilis (Varying ,, ) 2 



Metchnikoff has set down ten years as a great age for a rabbit, 

 and hence the record duration of nearly seven years for the 

 common hare is not high. The very low average duration shows a 

 poor viability. As there would seem to be no special difficulty in 

 providing Leporidse Avith conditions suitable to them, there is 

 probably some special reason for their low viability in captivity. 

 It is a remarkable circumstance that many of the most common 

 wild animals of Europe are difficult to keep in captivity. I think 

 it is quite possible that the fear of man which such animals have 

 acquired in their wild state, almost as a necessary condition of 

 their existence, by continuing to act after captivity, shortens the 

 life, often by accident, often by reaction of the mental state on 

 the general health. Animals from remoter parts of the world 

 have not an acquired intolerance of man to lose. 



Sicmmary of Rodentia. 



It is unlikely that animals in captivity reach the potential 

 longevities of their races, and it is a new and somewhat surprising 

 fact that Rodents live so long as these records show. There is 

 only a rough correspondence between maximum durations and 

 size, the figures amongst Rodents ranging from 20 years in the 

 Porcupine, 15 in some Squirrels, 13 in Marmots, 11 in Sousliks, 

 9 in Agoutis and Capybaras, down to 3 in Dormice. Metchnikoff 

 has shown {torn, cit.) that on the whole there is an inverse pro- 

 portion between the relative capacity of the large intestines and 

 the duration of life, animals with a capacious hind-gut usually 

 being shorter-lived in proportion to their size than those in which 

 the hind-gut is reduced. Rodents certainly present a marked 

 exception to his general rule, for in their case the hind-gut and 

 caecum are relatively very long and very capacious. The relatively 

 high viability of Rodents is another remarkable fact, and is most 

 striking in comparison with Insectivora. Their high specific 

 longevities and viabilities may be associated, I think, with the 



