Thus, though the mammal type of life is the characteristic of the Tertiary and post-Tertiary periods, it does not suddenly make its appearance without premonition in those periods.  Far back, in the Secondary, we find it under imperfect forms, struggling, as it were, to make good a foothold.  At length it gains a predominance under higher and better models.

So, too, of reptiles, the characteristic type of life of the Secondary period.  As we see in a dissolving view, out of the fading outlines of a scene that is passing away, the dim form of a new one emerging, which gradually gains strength, reaches its culmination, and then melts away in some other that is displacing it, so reptile-life doubtfully, appears, reaches its culmination, and gradually declines.  In all this there is nothing abrupt; the changes shade into each other by insensible degrees.

How could it be otherwise?  The hot-blooded animals could not exist in an atmosphere so laden with carbonic acid as was that of the primitive times.  But the removal of that noxious ingredient from the air by the leaves of plants under the influence of sunlight, the enveloping of its carbon in the earth under the form of coal, the disengagement of its oxygen, permitted their life.  As the atmosphere was thus modified, the sea was involved in the change; it surrendered a large part of its carbonic acid, and the limestone hitherto held in solution by it was deposited in the solid form.  For every equivalent of carbon buried in the earth, there was an equivalent of carbonate of lime separated from the sea —­not necessarily in an amorphous condition, most frequently under an organic form.  The sunshine kept up its work day by day, but there were demanded myriads of days for the work to be completed.  It was a slow passage from a noxious to a purified atmosphere, and an equally slow passage from a cold-blooded to a hot- blooded type of life.  But the physical changes were taking place under the control of law, and the organic transformations were not sudden or arbitrary providential acts.  They were the immediate, the inevitable consequences of the physical changes, and therefore, like them, the necessary issue of law.

For a more detailed consideration of this subject, I may refer the reader to Chapters I, ii., VII, of the second book of my “Treatise on Human Physiology,” published in 1856.

Is the world, then, governed by law or by providential interventions, abruptly breaking the proper sequence of events?

To complete our view of this question, we turn finally to what, in one sense, is the most insignificant, in another the most important, case that can be considered.  Do human societies, in their historic career, exhibit the marks of a predetermined progress in an unavoidable track?  Is there any evidence that the life of nations is under the control of immutable law?

May we conclude that, in society, as in the individual man, parts never spring from nothing, but are evolved or developed from parts that are already in existence?