470 DISEASE IN WILD MAMMALS AND BIRDS 
the same average for both groups. It seems then 
that tumors occur in animals in captivity longer 
than the average for their order, or in specimens 
that have the power to live under confined conditions until 
neoplasms develop. In this respect I recall the state- 
ments made by Harlow Brooks (5) that tumors will prob- 
ably be found more commonly in animals when they live 
in a manner comparable to that of urban man and that 
racial degeneracy will favor their development. There 
is adduced here perhaps the first definite evidence that 
long captivity allows tumor tendency to express itself 
but it does not prove that confinement increases tumors. 
Nor does the expectation of life, average or potential, 
stand in any direct and definite relationsliip to the fre- 
quency of neoplasms. The only clear case of long life 
and high tumor incidence is to be found in Parrots ; we feel 
however that some unknoAvn factor increases tumors of 
the renal-adrenal region in these birds and that unquali- 
fied statements about age and tumor growth are not per- 
missible. Since tumors grow in many wild-born 
specimens, a high percentage of which become known in 
the first few years of capti\dty, is it not highly probable 
that tumors are reasonably common in the wild and that 
we do not observe enough purely natural specimens to 
assume an immunity on the part of free living beasts. 
One of the undesirable features of captive breeding is 
consanguinity of parents and there is good reason to 
believe that tumor susceptibility can be bred into or out 
of a line of animals by mating tumor bearers and non- 
tumor bearers, the tendency following the rules of 
Mendelian inheritance (Slye). Is there any proof that 
inbreeding does not occur in the wild and if it do, it is 
perfectly possible that tumor tendency may be trans- 
mitted as a dominant character ; the effect of artificial or 
intentional inbreeding in captivity would only offer an 
opportunity for a summation of these influences. 
(5) Am. Jour. Med. 8oc., 1907, 133-769. 
