guides, men who are infected with disease, especially with tuberculosis. Only by preserving intact, as far as possible, the conditions under which their ancestry lived, and for which their evolution has therefore fitted them, can we hope to save for a time these—in many cases—noble and interesting races.
The decay of the races of the New World affords the most convincing, as it does the most terrible, proof that man's present evolution is mainly against zymotic disease. The extent to which that evolution has proceeded in the races of the Old World, as contrasted with those of the New, affords proof, moreover, of the immense antiquity of man's civilization in the former, for zymotic diseases, as we see, can have arisen only long after men had become very numerous and had begun to gather into settled communities; since before that time saprophytic micro-organisms would obviously have been unable to assume entirely parasitic ways of life in relation to him. The oldest histories extant tell of plagues and pestilences, of water and air-borne diseases, and when they were written earth-borne diseases, such as tuberculosis, were doubtless present also; yet since these, though death-dealing beyond the others—at least as regards tuberculosis—never prevail in a strikingly epidemic form, they are not mentioned.
We have now to consider man's evolution in relation to particular zymotic diseases, but here the proofs are often much less convincing than when we consider the proofs of his evolution in relation to zymotic disease in general. This again is just what might be expected, for since many zymotic diseases are not very prevalent or not very fatal, evolution against them is frequently not very marked; and it is only by comparing the peoples of countries in which they have been most prevalent and fatal with those of countries in which they have been least prevalent and fatal, or have been
