INTRODUCTION 29 
viability, up to twenty years. Anserine birds may live to 
be fifty, and, unless conditions are quite unfavorable, 
have a good resistance. Columbae may under good con- 
ditions live to be fifty. Gallinaceous birds may only be 
expected to survive twenty years, a figure also given for 
Fulicariae. Alectorides may live up to fifty years. 
Limicolag, though they do not thrive in captivity, may live 
thirty years. Impennes live poorly under artificial con- 
ditions, the greatest record being twelve years, a figure 
also holding for Crypturi. Struthiones, if the conditions 
be right, may live fifty years. 
Because of the variable specific longevities, it is fre- 
quently difficult to decide when an animal is senile. Man 
is said to be as old as his arteries, and his span of life 
nowadays is in the neighborhood of half a century. Par- 
rots exhibit lesions of the vascular system comparable 
to the arteriocapillary fibrosis of human beings, and their 
expected longevity is about the same or a little greater. 
From a study of our cases of this lesion in parrots it can 
be said to appear quite early in life and not to lead to 
organic disease as it is alleged to do in man. It is, how- 
ever, interesting to note that in those animals which are 
supposed to have the longest specific lives — elephants, 
snakes, anserine and raptatory birds, parrots — there is 
relatively low mortality and fewer infectious diseases 
are encountered. The last part of this statement should 
be qualified by stating that anserine birds and parrots 
are quite susceptible to mycoses, in all probability from 
musty food, which raises their death rate, but as 
this is accidental and artificial, it can be excluded 
from consideration. 
In a rough way there is a direct relationship between 
the size of an animal and its longevity, but this is not close 
enough to be a reliable guide ; whales and elephants live a 
long time, but so do snakes and parrots. Within orders 
this relation of size and expected longevity is more easily 
seen but is not absolute. I cannot state, according to my 
