ancestral lines will probably cease and retrogression ensue; unless, indeed, in a more enlightened era than our own, the Natural Selection of man is replaced by an Artificial Selection, which shall permit only the happy in mind and body to perpetuate the race.
But side by side with this evolution of brain and body along ancestral lines, which has engrossed the attention of biologists, but which now threatens to cease, man, during the last few thousand years, during which he has largely dwelt in towns and cities, has been undergoing another and a vastly important evolution in a different direction, an evolution which has escaped the observation of biologists, but which threatens by the survival of the fittest and the elimination of the unfittest to advance at accelerated speed in the near future; an evolution none the less real because it is intangible. If we would discover in what direction the evolution of an evolving species is proceeding, it is a good plan to note which members of it survive and which perish. We shall then, by observing the difference in qualities between the fit and the unfit, be able to discover which traits favour survival, and therefore which traits are undergoing evolution by the accumulation of inborn variations. Applying this test to man, it is manifest that the survivors in his species, they who attain to maturity and have offspring, who continue the race, are not, as in the remote past, necessarily the strong in limb and mind, but necessarily the strong against disease. The present evolution of man is therefore not mainly an evolution of physical or intellectual strength, as in his remote ancestry, but mainly an evolution against disease, and wherever men are crowded together, and can take disease from one another, or there arc other unfavourable circumstances, especially against zymotic disease—that is, disease due to or produced by living micro-organisms.
