Again, there is the danger of new “rays.” It is now a well-known fact that X-rays produce sterility when penetrating the human body in considerable strength or for any length of time. The effect can be guarded against by an armour of lead screens. X-rays are cut off by a thin sheet of lead. There are other rays, known as gamma-rays, which can penetrate several inches of lead. And higher up in the atmosphere, about seven miles up, another kind of radiation is found from which even a plate of lead five feet thick would be no protection. If the sun, entering an unknown part of space filled with denser matter, were to develop a form of radiation leading to a considerable increase in the penetrating atmospheric rays, the persistent action of these rays upon the human germ plasm might bring about the total destruction of the human race by reducing its birth-rate to zero. The effect might not even be discovered until it was too late to remedy it. And even if it were discovered in time, the action of the rays upon all life on the earth’s surface might have produced havoc enough to stop all food-supplies and produce universal starvation. It is difficult to see how even the greatest resources of science could meet such an emergency as that. But, short of such a new danger, there is little doubt that the resources of humanity will be able to meet all conceivable situations which may threaten it with destruction. In order to ensure the indefinite continuance of the human race on earth, it is necessary (1) to maintain adequate food supplies and (2) to conserve the procreative impulse. The latter condition might, indeed, be put first on the list. These are the conditions of bare existence. Progress and happiness are, as they always have been, secondary considerations. Many minds have regarded one or both of them as unessential. Thus, it is not at all necessary to contemplate an _increasing_ population of the globe. The leadership of the human race can much more effectively be maintained by educating individuals than by increasing their numbers. And as regards happiness, that can safely be left to take care of itself. All progress is progress towards greater happiness. Even mere existence can be a source of supreme happiness, as when a great danger has been successfully averted. * * * * * Once we are sure that the human race has a reasonable chance of indefinite survival, we can proceed to contemplate the changes, if any, which are likely to take place as the centuries and millennia roll on.