INTRODUCTION 27 
conceivable that the young and the very old might adapt 
themselves to new surromidings with much less readiness 
than the sturdy middle-aged adult. The age of animals 
upon arrival is very rarely known, and can only be 
recorded as "young," ''fully developed," and "old." 
This will have an effect upon statistics and when possible 
is noted in the text, but this is not practicable to the 
extent we desire. The meaning of "young," "adult," 
and "old" is not the same throughout the animal orders 
nor even within orders. 
Mitchell (3) has attempted to gain concrete ideas 
of the expectancy of life among animals by analyz- 
ing the records of the London Gardens. This gentleman 
bases his figures upon known ages and the length 
of time in captivity, from a combination of which 
data the specific viability and the potential longevity 
may be estimated. Such results, he admits, can 
only be approximate and they show Avithin classes and 
orders, a decided lack of uniformity. The terms "spe- 
cific" and "potential" longevity, coined by Sir Ray 
Lankester, apply, for the first, to the average length of 
life as it is affected by external conditions and those 
incident to procreation, while, if an animal be under ideal 
conditions it will attain the potential longevity which is 
longer than the former. These considerations have a 
biological and economic importance, while a knowledge 
of the pathology shown by the various groups may help 
to explain these durations of life. Contrariwise figures 
of the expected longevity may assist us in evaluating 
youth and senility in the causes of death but can hardly 
affect the comparative nature of the lesion. 
A resume of Mitchell's studies indicates that the 
higher apes have a potential longevity and a hardihood 
much less than man but still upwards of thirty years. As 
one investigates lower in monkeys, life periods become 
shorter, while in the next ord er, Lemures, the length of 
(3) Proceedings, Zoological Society, London, 1911, p. 425. 
