the moderate supposition that each generation of coal plants took ten years to come to maturity—­then, each foot-thickness of coal represents five hundred years.  The superimposed beds of coal in one coal-field may amount to a thickness of fifty or sixty feet, and therefore the coal alone, in that field, represents 500 x 50 = 25,000 years.  But the actual coal is but an insignificant portion of the total deposit, which, as has been seen, may amount to between two and three miles of vertical thickness.  Suppose it be 12,000 feet—­which is 240 times the thickness of the actual coal—­is there any reason why we should believe it may not have taken 240 times as long to form?  I know of none.  But, in this case, the time which the coal-field represents would be 25,000 x 240 =6,000,000 years.  As affording a definite chronology, of course such calculations as these are of no value; but they have much use in fixing one’s attention upon a possible minimum.  A man may be puzzled if he is asked how long Rome took a-building; but he is proverbially safe if he affirms it not to have been built in a day; and our geological calculations are all, at present, pretty much on that footing.

A second consideration which the study of the coal brings prominently before the mind of anyone who is familiar with palaeontology is, that the coal Flora, viewed in relation to the enormous period of time which it lasted, and to the still vaster period which has elapsed since it flourished, underwent little change while it endured, and in its peculiar characters, differs strangely little from that which at present exists.

The same species of plants are to be met with throughout the whole thickness of a coal-field, and the youngest are not sensibly different from the oldest.  But more than this.  Notwithstanding that the carboniferous period is separated from us by more than the whole time represented by the secondary and tertiary formations, the great types of vegetation were as distinct then as now.  The structure of the modern club-moss furnishes a complete explanation of the fossil remains of the Lepidodendra, and the fronds of some of the ancient ferns are hard to distinguish from existing ones.  At the same time, it must be remembered, that there is nowhere in the world, at present, any forest which bears more than a rough analogy with a coal-forest.  The types may remain, but the details of their form, their relative proportions, their associates, are all altered.  And the tree-fern forest of Tasmania, or New Zealand, gives one only a faint and remote image of the vegetation of the ancient world.

Once more, an invariably-recurring lesson of geological history, at whatever point its study is taken up:  the lesson of the almost infinite slowness of the modification of living forms.  The lines of the pedigrees of living things break off almost before they begin to converge.