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 on 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. [Sidenote: Universal control of physical agents over organisms.] To this doctrine of the control of physical agencies over organic forms I acknowledge no exception, not even in the case of man. The varied aspects he presents in different countries are the necessary consequences of those influences. [Sidenote: The case of man.] He who advocates the doctrine of the unity of the human race is plainly forced to the admission of the absolute control of such agents over the organization of man, since the originally-created type has been brought to exhibit very different aspects in different parts of the world, apparently in accordance with the climate and other purely material circumstances. To those circumstances it is scarcely necessary to add manner of life, for that itself arises from them. The doctrine of unity demands as its essential postulate an admission of the paramount control of physical agents over the human aspect and organization, else how could it be that, proceeding from the same stock, all shades of complexion in the skin, and variety in the form of the skull, should have arisen? Experience assures us that these are changes assumed only by slow degrees, and not with abruptness; they come as a cumulative effect. They plainly enforce the doctrine that national type is not to be regarded as a definite or final thing, a seeming immobility in this particular being due to the attainment of a correspondence with the conditions to which the type is exposed. Let those conditions be changed, and it begins forthwith to change too. I repeat it, therefore, that he who receives the doctrine of the unity of the human race, must also accept, in view of the present state of humanity on various parts of the surface of our planet, its necessary postulate, the complete control of physical agents, whether natural, or arising artificially from the arts of civilization and the secular progress of nations toward a correspondence with the conditions to which they are exposed. To the same conclusion also must he be brought who advocates the origin of different races from different centres. It comes to the same thing, whichever of those doctrines we adopt. Each brings us to the admission of the transitory nature of typical forms, to their transmutations and extinctions. [Sidenote: Human variations.]