92 J. B. CLELAND. 
which members are exposed to certain forms of 
chronic irritation. These three factors require a little fur- 
ther explanation. As regards the number of individuals, 
since cancer only appears in a small percentage, instances 
of the disease would be more likely to be encountered when 
the species was abundant in members and these easily acces- 
sible to intelligent observation. Thus many cases are recog- 
nised amongst the millions of civilized man, but fewer pro- 
portionately amongst savage races, partly because the latter 
are less closely under medical observation, though also be- 
cause they have, on an average, a shorter life, and are’ less 
exposed to certain forms of chronic irritation. With Dr. 
Bancroft? I have reported a case of carcinoma of the liver 
in an Australian aboriginal. Rats and mice are very abun- 
dant in numbers, though being elusive and small, they are 
not very closely under human observation. When examined 
systematically one would expect to find, and actually does 
find, cases of cancer amongst wild rats and mice. As large 
numbers of tame rats and mice are bred, amongst these 
also, from time to time, cases of cancer are found, and, in 
fact, such spontaneous tumours have yielded important ex- 
perimental data at the hands of cancer workers. Lions and 
tigers are relatively fewer in numbers—nevertheless I have 
met with a case of carcinoma of the mammary gland in 
an old lioness in the Zoological Gardens in Perth, W.A. 
This case, however, is probably unique. It is only to be 
expected, therefore, that the more abundant the individuals 
of a species, the more likely are we to meet with cases of 
malignant growths. 
The average span of life of the individuals as compared 
with the natural length of life might be better put as the 
number of indwiduals who reach old age. By the natural 
length of life is meant that age at which senescence de- 
tAust. Med. Gaz., May, 1913, p. 465. 
