222 THE SCIENTIFIC MONTHLY
There are certain tumors which grow in a new host of the same species for some time until, as a result of this growth, certain sub- stances are produced by the host which cause the tumor fbrst to stop growing and later to retrogress, become smaller, and finally to disap- pear. Other tumors may grow continuously without disappearing if transplanted into related strains of the same species. But if they are transplanted into different strains then they either can not grow at all or after temporary growth begin to get smaller and disappear. Thus it has been possible to demonstrate by experimental methods that there are fine chemical differences not only between different species and be- tween different individuals of the same species, but also between differ- ent sets of families which constitute a strain, for certain chemical char- acters differentiate them from other strains of the same species. It has been shown, for instance, that white mice bred in Europe differ chemically from white mice bred in America, although the appearance of both strains may be identical. If we transplant an inoculable tumor of an American white mouse into a European strain, we find that such a tumor does not grow or that after a period of growth it becomes smaller, while in the American mice it does grow.
That certain chemical substances may become actiye to protect tilie new host against a transplanted tumor even if it does grow continuonsly can be shown by means of a second inoculation of the same kind of tumor and the same individual. We usually find that the growth of the first tumor has some inhibiting effect on the growth of the second tumor, but this inhibiting effect differs with different tumors. We must therefore assume that different tumors after transplantation into other animals of the same species contain a substance which character- izes the individual from which the tumor cells are derived more or less in the same way as the normal cells of an individual do, and that this differential substance calls forth protective mechanisms on the part of the host. Some tumors possess this differential substance to a lesser degree than others, or they are less sensitive to the defensive mechanism which is called forth in the host and they can therefore be more readily transplanted into other individuals.
As we stated before, it is possible to increase artificially the extent of this defensive reaction on the part of the host. We can accomplish this in a way similar to the one we used in the case of inoculation of bacteria. If we wish to produce a substance which is protective against a certain bacterium, we inject small quantities of dead bacterial sub- stances or of the living bacteria after their virulence has been de- creased, into the animal. Under the infiuence of these vaccines, as they are called, a certain immunity is frequently produced in the in- dividual. If later we inject the fuUy virulent organism iuto the animal, it is often able to withstand the infection to which an animal not so treated would succumb. Now in the case of cancer the hostile agents
�� �
