of
the exposure to the salt water or the amount of salt added, a point
can be reached where some, but not all, of the amoebae are destroyed.
Whether few or many survive depends upon the degree of injury
produced. Much the same phenomena can be produced by gradually heating
the water in which the amoebae are contained. It is even possible
gradually to accustom such small organisms to an environment which
would destroy them if suddenly subjected to it, but in the process of
adaptation many individuals will have perished.

It is evident from such an experiment that when a living organism is
subject to an environment to which it has not become adapted and which
is unfavorable, such alterations in its structure may be produced that
it is incapable of living even when it is again returned to the
conditions natural to it. Such alterations of structure or injuries
are called the _lesions_ of disease. We have seen that in certain
individuals the injury was sufficient to inhibit for a time only the
usual manifestations of life; these returned when the organism was
removed from the unfavorable conditions, and with this or preceding it
the organisms, if visibly altered, regained the usual form and
structure. We may regard this as disease and recovery. In the disease
there is both the injury or lesion and the derangement of vital
activity dependent upon this. The cause of the disease acted on the
organism from without, it was external to it. Whether the injurious
external conditions act as in this case by a change in the surrounding
osmotic pressure, or by the destruction of ferments within the cell,
or by the introduction into the cell of substances which form stable
chemical union with certain of its constituents, and thus prevent
chemical processes taking place which are necessary for life, the
result is the same.

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