many. Unfavorable conditions which will destroy all individuals of a
species exposed to them must be extremely rare.[1] There is no such
individuality in non-living things. In a mass of sugar grains each
grain shows just the same characteristics and reacts in exactly the
same way as all the other grains of the mass. Individuality, however
expressed, is due to structural variation. It is almost impossible to
conceive in the enormous complexity of living things that any two
individuals, whether they be single cells or whether they be formed of
cell masses, can be exactly the same. It is not necessary to assume in
such individual differences that there be any variation in the amount
and character of the component elements, but the individuality may be
due to differences in the atomic or molecular arrangements. There are
two forms of tartaric-acid crystals of precisely the same chemical
formula, one of which reflects polarized light to the left, and the
other to the right. All the left-sided crystals and all the
right-sided are, however, precisely the same. 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