Up to the present it has been assumed that the environment of the amoeba is that to which it has become adapted and which is favorable to its existence.  Under these conditions its structure conforms to the type of the species, as do also the phenomena which it exhibits, and it can assimilate food, grow and multiply.  If, during the observation, a small crystal of salt be placed in the fluid, changes almost instantly take place.  Motion ceases, the amoebae appear to shrink into smaller compass, and they become more granular and opaque.  If they remain a sufficiently long time in this fluid, they do not regain their usual condition when placed again in fresh water.  None of the phenomena which characterized the living amoebae appear:  we say they are dead.  After a time they begin to disintegrate, and the bacteria contained in the water and on which the amoebae fed now invade their tissue and assist in the disintegration.  By varying the duration 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.