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?
If any one should object to or deride the doctrine of the evolution or successive development of the animated forms which constitute that unbroken organic chain reaching from the beginning of life on the globe to the present times, let him reflect that he has himself passed through modifications the counterpart of those he disputes.

For nine months his type of life was aquatic, and during that time he assumed, in succession, many distinct but correlated forms.

At birth his type of life became aerial; he began respiring the atmospheric air; new elements of food were supplied to him; the mode of his nutrition changed; but as yet he could see nothing, hear nothing, notice nothing.