The philosophy of life herein flashed out by the poet was later to be elaborated fully on its objective or observational side by Spencer--the philosopher par excellence of evolution--and finally, also, of course, on the objective side, to become an assured fact of science through the publication in 1859 of Darwin's epoch-making book, "The Origin of Species," wherein the laws, so disturbing to many at the time, of natural selection and the survival of the fittest were fully set forth. While the genetic view of nature, as the phraseology of to-day goes, had been anticipated in writers on cosmology like Leibnitz and Laplace, in geology by such men as Hutton and Lyall, and had entered into the domain of embryology through the researches of Von Baer, and while Spencer had already formulated a philosophy of evolution, Darwin went out into the open and studied the actual facts in the domain of living beings. His studies made evolution a certainty. They revealed the means by which its processes were accomplished, and in so doing pointed to an origin of man entirely opposed to orthodox views upon this subject. Thus was inaugurated the last great phase in the struggle between mind and spirit. Henceforth, science stood completely revealed as the unflinching searcher of truth. Intuition was but a handmaid whose duty was to formulate working hypotheses, to become scientific law if provable by investigation or experiment, to be discarded if not. The aspects which this battle has assumed in the latter half of the century have been many and various. Older sciences with a new lease of life and sciences entirely new have advanced along the path pointed out by the doctrines of evolution. Battalions of determined men have held aloft the banner of uncompromising truth. Each battalion has stormed truth's citadel only to find that about its inmost reality is an impregnable wall. The utmost which has been attained in any case is a working hypothesis, useful in bringing to light many new objective phenomena, it is true, but, in the end, serving only to deepen the mystery inherent in the nature of all things. Such a working hypothesis was the earlier one of gravitation whose laws of action were elaborated by Sir Isaac Newton, and by the great mind of Laplace were still further developed with marvelous mathematical precision in his "Méchanique Celeste." Such another hypothesis is that of the atomic theory of the constitution of matter usually associated with the name of Dalton, though it has undergone many modifications from other scientific thinkers. Of this hypothesis Theodore Merz writes in his history of nineteenth-century scientific thought: