PRESIDENTIAL ADDRESS. 93 
velops to such an extent as to lead to extinction of life. In 
most individuals of a species, accident or disease leads to 
death before senescence can play its part. Thus, in man, 
the average expectation of life at any age falls far short 
of the expectation of life if mere senescence had alone 
to be considered. The natural length of life varies, of 
course, markedly in different species, being probably a year 
or so in wild rats and mice, and about 70 years in man. 
But not only does it vary in extent according to the species, 
but it varies, though to a much less extent, according to the 
family, and even, probably, according to the individual. 
Long-lived human stocks and short-lived ones are well- 
known. I have seen a man of about 70 with the aspect of 
one of 40, and probably with his expectation of life, whilst 
the son of 40 looked like a lad of 20. Thus the natural 
length of life of the individual, or the age at which the 
animal machine will go to pieces from natural wear and 
tear, apart from being cut short by accident or disease, 
varies markedly with the species, decidedly with the family 
and somewhat with the individual. The greater the num- 
ber of individuals of the species who reach old age, and thus 
approximate to the tether of their natural existence, the 
sreater will be the average span of life of the individuals 
of the species. Now cancer is essentially a disease of old 
age. Even in those forms that occur in early life, it is note- 
worthy that they especially arise in cells which may be 
looked on as approaching senescence, or as having undergone 
approximately the number of mitotic divisions that would 
naturally be expected of them. Cancer of the liver and 
certain sarcomata in young children are instances in point. 
In such young persons, one never sees epithelioma of the 
lip or tongue, or carcinoma of the breast tissues, in which 
the cells will continue mitosing for many years to come. 
Hence one finds that, be it man or lower vertebrates, the 
greater the number of individuals who reach old age, the 
