Even on land it is possible to find creatures that with a little modification might become excessively dangerous to the human ascendency. Most people have read of the migratory ants of Central Africa, against which no man can stand. On the march they simply clear out whole villages, drive men and animals before them in headlong rout, and kill and eat every living creature they can capture. One wonders why they have not already spread the area of their devastations. But at present no doubt they have their natural checks, of ant-eating birds, or what not. In the near future it may be that the European immigrant, as he sets the balance of life swinging in his vigorous manner, may kill off these ant-eating animals, or otherwise unwittingly remove the checks that now keep these terrible little pests within limits. And once they begin to spread in real earnest, it is hard to see how their advance could be stopped. A world devoured by ants seems incredible now, simply because it is not within our experience; but a naturalist would have a dull imagination who could not see in the numerous species of ants, and in their already high intelligence, far more possibility of strange developments than we have in the solitary human animal. And no doubt the idea of the small and feeble organism of man, triumphant and omnipresent, would have seemed equally incredible to an intelligent mammoth or a palæolithic cave bear.
And, finally, there is always the prospect of a new disease. As yet science has scarcely touched more than the fringe of the probabilities associated with the minute fungi that constitute our zymotic diseases. But the bacilli have no more settled down into their final quiescence than have men; like ourselves, they are adapting themselves to new conditions and acquiring new powers. The plagues of the Middle Ages, for instance, seem to have been begotten of a strange bacillus engendered under conditions that sanitary science, in spite of its panacea of drainage, still admits are imperfectly understood, and for all we know even now we may be quite unwittingly evolving some new and more terrible plague--a plague that will not take ten or twenty or thirty per cent., as plagues have done in the past, but the entire hundred.