The experiments with the amoebae show also two of the most striking characteristics of living matter. 1.  It is adaptable.  Under the influence of unusual conditions, alterations in structure and possibly in substance, may take place, in consequence of which the organisms under such external conditions may still exhibit the usual phenomena.  The organism cannot adapt itself to such changes without undergoing change in structure, although there may be no evidence of such changes visible.  This alteration of structure does not constitute a disease, provided the harmonious relation of the organism with the environment be not impaired.  An individual without a liver should not be regarded as diseased, provided there can be such an internal adjustment that all of the vital phenomena could go on in the usual manner without the aid of this useful and frequently maligned organ. 2.  It is individual.  In the varying degrees of exposure to unfavorable conditions of a more serious nature some, but not all, of the organisms are destroyed; in the slight exposure, few; in the longer, 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