s generally assumed,
combine to obscure the evolution of the insect, and we must be content
for the present with these general indications. The vast unexplored
regions of Africa, South America, and Central Australia, may yet yield
further clues, and the riddle of insect-metamorphosis may some day
betray the secrets which it must hold. For the moment the Carboniferous
insects interest us as a rich material for the operation of a coming
natural selection. On them, as on all other Carboniferous life, a great
trial is about to fall. A very small proportion of them will survive
that trial, and they trill be the better organised to maintain
themselves and rear their young in the new earth.

The remaining land-life of the Coal-forest is confined to worm-like
organisms whose remains are not preserved, and land-snails which do
not call for further discussion. We may, in conclusion, glance at the
progress of life in the waters. Apart from the appearance of the
great fishes and Crustacea, the Carboniferous period was one of great
stimulation to aquatic life. Constant changes were taking place in the
level and the distribution of land and water. The aspect of our coal
seams to-day, alternating between thick layers of sand and mud, shows
a remarkable oscillation of the land. Many recent authorities have
questioned whether the trees grew on the sites where we find them
to-day, and were not rather washed down into the lagoons and shallow
waters from higher ground. In that case we could not too readily imagine
the forest-clad region sinking below the waves, being buried under the
deposits of the rivers, and then emerging, thousands of years later, to
receive once more the thick mantle of sombre vegetation. Probably
there was less rising and falling of the crust than earlier geologists
imagined. But, as one of the most recent and most critical authorities,
Professor Chamberlin, observes, the comparative purity of the coal, the
fairly uniform thickness of the seams, the bed of clay representing
soil at their base, the frequency with which the stumps are still found
growing upright (as in the remarkable exposed Coal-forest surface in
Glasgow, at the present ground-level), [*] the perfectly preserved fronds
and the general mixture of flora, make it highly probable that the
coal-seam generally marks the actual site of a Coal-forest, and there
were considerable vicissitudes in the distribution of land and water.
Great areas of land repeatedl