It is often seen that in households where all members are exposed to the same danger, or again in schools or troops where everyone lives the same life, disease does not strike everyone indifferently.
The Greek conception of a life in harmony with nature found its most complete development in the rationalism of the Renaissance and of the centuries that followed it.
It is, of course, quite natural that a biologist whose attention had been aroused by noticing in his own case the phenomena of precocious old age should turn to study the causes of it.
Disease is not the prerogative of man and the domestic animals, so it was quite natural to see if the lower animals, with very simple organizations, showed pathological phenomena, and if so, infection, cure and immunity could be observed among them.
The appearance of aged persons is too well known to make detailed description necessary. The skin of the face is dry and wrinkled and generally pale. The hairs on the head and the body are white. The back is bent, and the gait is slow and laborious, whilst the memory is weak. Such are the most familiar traits of old age.
Most of the centenarians whom I have been able to see have been so defective mentally that all that can be studied in them are the physical qualities and functions.
