The number of possible variations in the chemical structure of a substance so complex as is protoplasm is inconceivable.
In no way is the individuality of living matter more strongly expressed than in the resistance to disease.

The variation in the degree of resistance to an unfavorable environment is seen in every tale of shipwreck and exposure.

In the most extensive epidemics certain individuals are spared; but here care must be exercised in interpreting the immunity, for there must be differences in the degree of exposure to the cause of the epidemic.

It would not do to interpret the immunity to bullets in battle as due to any individual peculiarity, save possibly a tendency in certain individuals to remove the body from the vicinity of the bullets; in battle and in epidemics the factors of chance and of prudence enter.

No other living organism is so resistant to changes in environment as is man, and to this resistance he owes his supremacy.