beset with, mystery. We talk glibly of aqueous deposit as the physical origin of stratification; but we know little indeed of the physical conditions under which this agency worked in early times. The scientific naturalists of the Challenger Expedition report as the result of their investigations that nowhere in the existing world of waters have they found going on anywhere such deposits as are necessary to account for the vast massive accumulations of the Palaeozoic sandstones.
Before such, mountains as those of the Cambrian formation on the northwest coast of Scotland—cut out of the thickness of apparently one continuous deposit—full of the ripple-marks of the sea, and yet destitute of life—the theoretical uniformitarian may well stand abashed. Similar difficulties are crowded into the conditions under which our great storages of carbon were provided for by repeated elevations and depressions of the land, each elevation giving occasion for the growth of a dense and rich vegetation; and each depression potting it up and preserving it for future use. Similar difficulties beset the equally massive limestone formations of the Secondary rocks. But even these difficulties are less serious and less profound than those which beset the progress of organic life. Only, in this case there are some great outlines which are clear and definite. We can see that organic life has advanced from less to more—from low to higher levels—from the generalized to the specialized, and from various functions performed roughly by some one rude and simple mechanism, to the same functions separated, elevated, and committed to the care of selected and adapted organs. We can see how there is some strange but profound analogy between this magnificent line of march and that along which every living creature goes in its individual growth. Just as the science of embryology has in in some measure revealed to us how—that is, in what order—"the bones do grow in the womb of her that is with child," so in the embryology of this planet, as revealed to us in the rocks, we can see the steps of a process which is not only analogous but homologous. That is to say, the two pathways are not only vaguely like each, other according to some dim resemblance, but are identical as corresponding parts in one plan, and of one intellectual method. We can see that the past ages were full of prophetic germs. We can see the rise, one after another, of structures which were incipient, useless, or comparatively useless for a time, but destined in the future for some splendid service. Our physiologists, and anatomists, and morphologists are wholly unable to resist this evidence when it is their business to describe the facts. The structure of their own mind compels them to admit it, even when they struggle hard to shut their eyes against it.
Few men have used language more expressive of conceptions
