It may be a century, or it may be more than five centuries, before the matter would, if let alone, force itself upon a desperate humanity, brutalized by over-crowding, and the struggle for food. A return to Nature’s terrible selection of the fittest may, it is conceivable, be in this way in store for us. But it is more probable that humanity will submit, before that condition occurs, to a restriction by the community in respect of the right to multiply, with as good a grace as it has given up the right to murder and to steal. In view of this Man must, in entering on his kingdom, at once proceed to perfect those studies as to the transmission of qualities by heredity which have as yet been only roughly carried out by breeders of animals and horticulturists. There is absolutely no provision for this study in any civilized community, and no conception among the people or their leaders that it is a matter which concerns any one but farmers. 16. AN UNTOUCHED SOURCE OF ENERGY. The applications of steam and electricity have so far astonished and gratified the rebel Man, that he is sometimes disposed to conclude that he has come to the end of his power of relieving himself from the use of his own muscles for anything but refined movements and well-considered health-giving exercises. One of the greatest of chemical discoverers at this time living, M. Berthelot, has, however, recently pressed on our attention the question of the possibility of tapping the central heat of the earth and making use of it as a perennial source of energy. Many competent physicists have expressed the opinion that the mechanical difficulties of such a boring, as would be necessary, are insuperable. No one, however, would venture to prophesy, in such a matter as this, that what is prevented by insuperable obstacles to-day may not be within our powers in the course of a few years. 17. SPECULATIONS AS TO THE MARTIANS.