Such facts remind us how far more comprehensive the results of science will become when the different branches of scientific investigation are pursued in connection with each other.  When chemists have brought their knowledge out of their special laboratories into the laboratory of the world, where chemical combinations are and have been through all time going on in such vast proportions,—­when physicists study the laws of moisture, of clouds and storms, in past periods as well as in the present,—­when, in short, geologists and zoologists are chemists and physicists, and vice versa,—­then we shall learn more of the changes the world has undergone than is possible now that they are separately studied.

It may be asked, how any clue can be found to phenomena so evanescent as those of clouds and moisture.  But do we not trace in the old deposits the rainstorms of past times?  The heavy drops of a passing shower, the thick, crowded tread of a splashing rain, or the small pinpricks of a close and fine one,—­all the story, in short, of the rising vapors, the gathering clouds, the storms and showers of ancient days, we find recorded for us in the fossil rain-drops; and when we add to this the possibility of analyzing the chemical elements which have been absorbed into the soil, but which once made part of the atmosphere, it is not too much to hope that we shall learn something hereafter of the meteorology even of the earliest geological ages.

The peculiar character of the vegetable tissue in the trees of the Carboniferous period, containing, as it did, a large supply of resin drawn from the surrounding elements, confirms the view of the atmospheric conditions above stated; and this fact, as well as the damp, soggy soil in which the first forests must have grown, accounts for the formation of coal in greater quantity and more combustible in quality than is found in the more recent deposits.  But stately as were those fern forests, where plants which creep low at our feet to-day, or are known to us chiefly as underbrush, or as rushes and grasses in swampy grounds, grew to the height of lofty trees, yet the vegetation was of an inferior kind.

There has been a gradation in time for the vegetable as well as the animal world.  With the marine population of the more ancient geological ages we find nothing but sea-weeds,—­of great variety, it is true, and, as it would seem, from some remains of the marine Cryptogams in early times, of immense size, as compared with modern sea-weeds.  But in the Carboniferous period, the plants, though still requiring a soaked and marshy soil, were aerial or atmospheric plants:  they were covered with leaves; they breathed; their fructification was like that which now characterizes the ferns, the club-mosses, and the so-called “horse-tail plants,” (Equisetaceae,) those grasses of low, damp grounds remarkable for the strongly marked articulations of the stem.