In the early days of geological science when observers were few, great groups of strata were arranged under an artificial classification, which, while it has lost to a certain extent the specific value which it then assumed to possess, is still retained for purposes of convenient reference. Masters of the science acquired, so to say, a possessive interest in certain regions of it, and the names of Sedgwick, Murchison, Jukes, Phillips, Lyell, and others became, and will remain, inseparably associated with the history of those great divisions of the materials of the earth's crust, which, under the names of the Cambrian, Silurian, Devonian, Carboniferous, and Tertiary formations, have become familiar to us. In those days, when observations were limited to a comparatively small area, the lines separating most of these formations were supposed to be hard and definite; forms of life which characterized one, were presumed to have become entirely extinct before the inauguration of those which succeeded them, and breaks in the stratigraphical succession appeared to justify the opinion, held by a large and influential section, that great cataclysms or catastrophes had marked the time when one age or formation terminated and another commenced to succeed it. By degrees, and with the increase of observers, both in England and in every portion of the world, modifications of these views obtained; passage beds were discovered, connecting by insensible gradations formations which had hitherto been supposed to present the most abrupt separations; transitional forms of life connecting them were unearthed; and an opinion was advanced, and steadily confirmed, which at the present day it is probable no one would be found to dispute, that not all in one place or country, but discoverable in some part or other of the world, a perfect sequence exists, from the very earliest formations of which we have any cognizance, up to the alluvial and marine deposits in process of formation at the present day.[71] Correlatively it was deduced that the same phenomena of nature have been in action since the earliest period when organic existence can be affirmed. The gradual degradation of pre-existing continents by normal destructive agencies, the upheaval and subsidence of large areas, the effusion from volcanic vents, into the air or sea, of ashes and lavas, the action of frost and ice, of heat, rain, and sunshine--all these have acted in the past as they are still acting before our eyes. In earlier days, arguing from limited data, a progressive creation was claimed which confined the appearance of the higher form of vertebrate life to a successive and widely-stepped gradation. Hugh Miller, and other able thinkers, noted with satisfaction the appearance, first of fish, then of reptiles, next of birds and mammals, and finally, as the crowning work of all, both geologically and actually, quite recently of man.