'The present invariability of the world of organization is the direct consequence of the physical equilibrium, and so it will continue as long as the mean temperature, the annual supply of light, the composition of the air, the distribution of water, oceanic and atmospheric currents, and other such agencies, remain unaltered; but if any one of these, or of a hundred other incidents that might be mentioned, should suffer modification, in an instant the fanciful doctrine of the immutability of species would be brought to its true value. The organic world appears to be in repose, because natural influences have reached an equilibrium. A marble may remain forever motionless upon a level table; but let the surface be a little inclined, and the marble will quickly run off. What should we say of him who, contemplating it in its state of rest, asserted that it was impossible for it ever to move? 'When, therefore, we notice such orderly successions, we must not at once assign them to a direct intervention, the issue of wise predeterminations of a voluntary agent; we must first satisfy ourselves how far they are dependent upon mundane or material conditions, occurring in a definite and necessary series, ever bearing in mind the important principle that an orderly sequence of inorganic events necessarily involves an orderly and corresponding progression of organic life. 'To this doctrine of the control of physical agencies over organic forms I acknowledge no exceptions, not even in the case of man. The varied aspects he presents in different countries are the necessary consequences of those influences.' Whether we advocate the doctrine of the origination of the human race from a single pair, or from different races at different centres, we are, in Dr. Draper's judgment, alike driven to the conclusion of the transitory nature of typical forms, to their transmutations and extinctions. In the former case, we can only account for diverse races, having different shades of complexion, different varieties of skull, etc., by the admission of the paramount control of physical agents, such as climate and other purely material circumstances; in the latter, we can only account for the varieties visible among the different races themselves on similar grounds.