To the modern naturalist, vegetable life, with regard to its uses, is the great accumulator of pabulum for the sustenance of the higher forms of vital energy manifested in the animal. In the Palæozoic this consideration sinks in importance. In the Coal period we know few land animals, and these not vegetable feeders, with the exception of some insects, millipedes, and snails. But the Carboniferous forests did not live in vain, if their only use was to store up the light and heat of those old summers in the form of coal, and to remove the excess of carbonic acid from the atmosphere. In the Devonian period even these utilities fail, for coal does not seem to have been accumulated to any great extent, though the abundant petroleum of the Devonian is, no doubt, due to the agency of aquatic vegetation. In addition to scorpions, a few insects are the only known tenants of the Devonian land, and these are of kinds whose lame probably lived in water, and were not dependent on land plants. We may have much yet to learn of the animal life of the Devonian; but for the present, the great plan of vegetable nature goes beyond our measures of utility; and there remains only what is perhaps the most wonderful and suggestive correlation of all, namely, that our minds are able to trace in these perished organisms structures similar to those of modern plants, and thus to reproduce in imagination the forms and habits of growth of living things which so long preceded us on the earth.
In another way Huxley has put the utilitarian aspect of the case so admirably, that I cannot refrain from quoting his clever apotheosis of nature in connection with the production of coal.
"Nature is never in a hurry, and seems to have had always before her eyes the adage, 'Keep a thing long enough, and you will find a use for it.' She has kept her beds of coal for millions of years without being able to find a use for them; she has sent them beneath the sea, and the sea beasts could make nothing of them; she had raised them up into dry land, and laid the black veins bare, and still for ages and ages there was no living thing on the face of the earth that could see any sort of value in them; and it was only the other day, so to speak, that she turned a new creature out of her workshop, who, by degrees, acquired sufficient wits to make a fire, and then to discover that the black rock would burn.