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.

This wonderful confirmation of the Biblical history of creation appealed so gratefully to many, that it caused for a time a disposition to cramp discovery, and even to warp the facts of science, in order to make them harmonize with the statements of Revelation. The alleged proofs of the existence of pre-historic man were for a long time jealously disputed, and it was only by slow degrees that they were admitted, that the tenets of the Darwinian school gained ground, and that the full meaning was appreciated of such anomalies as the existence at the present day of Ganoid fishes both in America and Europe, of true Palæozoic type, or of Oolitic forms on the Australian continent and in the adjacent seas.

But step by step marvellous palæontological discoveries were made, and the pillars which mark the advent of each great form of life have had to be set back, until now no one would, I think, be entirely safe in affirming that even in the Cambrian, the oldest of all fossiliferous formations, vestiges of mammals, that is to say, of the highest forms of life, may not at a future day be found, or that the records contained between the Cambrian and the present day, may not in fact be but a few pages as compared with the whole volume of the world’s history.[72]

It is with the later of these records that we have to deal, in which discoveries have been made sufficiently progressive to justify the expectation that they have by no means reached their limit, and sufficiently ample in themselves to open the widest fields for philosophic speculation and deduction.

Before stating these, it may be premised that estimates have been attempted by various geologists of the collective age of the different groups of formations.[73] These are based on reasonings which for the most part it is unnecessary to give in detail, in so much as these can scarcely yet be considered to have passed the bounds of speculation, and very different results can be arrived at by theorists according to the relative importance which they attach to the data employed in the calculation.

Thus Mr. T. Mellard Reade, in a paper communicated to the Royal Society in 1878, concludes that the formation of the sedimentary strata must have occupied at least six hundred million years: which he divides in round numbers as follows:—

Millions of Years.
Laurentian, Cambrian, and Silurian 200
Old Red, Carboniferous, Permian, and New Red 200
Jurassic, Wealden, Cretaceous, Eocene, Miocene,
Pliocene, and Post Pliocene		 200
600

He estimates the average thickness of the sedimentary crust of the earth to be at least one mile, and from a computation of the proportion of carbonate and sulphate of lime to materials held in suspension in various river-waters from a variety of formations, infers that one-tenth of this crust is calcareous.

He estimates the annual flow of water in all the great river-basins, the proportion of rain-water running off the granitic and trappean rocks, the percentage of lime in solution which they carry down, and arrives at the conclusion that the minimum time requisite for the elimination of the calcareous matter contained in the sedimentary crust of the earth, is at least six hundred millions of years.