A farther objection is the uselessness of the existence of plants for a long period, without any animals to subsist on or enjoy them, and even without forming any accumulation of fossil fuel or other products useful to man. The only direct answer to this has already been given. The previous existence of plants may have been, and probably was, essential to the comfort and subsistence of the animals afterwards introduced. Independently of this, however, we have an analogous case in the geological history of animals, which prevents this fact from standing alone. Why was the earth tenanted so long by the inferior races of animals, and why were so much skill and contrivance expended on their structures, and even on their external ornament, when there was no intelligent mind on earth to appreciate their beauties. Even in the present world we may as well ask why the uninhabited islands of the ocean are found to be replete with luxuriant vegetable life, why God causes it to rain in the desert where human foot never treads, or why he clothes with a marvellous exuberance of beautiful animal and plant forms the depths of the sea. We can but say that these things seemed and seem good to the Creator, and may serve uses unknown to us; and this is precisely what we must be content to say respecting the plant-creation of the Eozoic period. Some writers[89] on this subject have suggested that the cosmical use of this plant-creation was the abstraction from the atmosphere of an excess of carbonic acid unfavorable to the animal life subsequently to be introduced. This use it may have served, and when its effects had been gradually lost through metamorphism and decay, that second great withdrawal of carbon which took place in the Carboniferous period may have been rendered necessary. The reasons afforded by natural history for supposing that plants preceded animals are thus stated by Professor Dana: "The proof from science of the existence of plants before animals is inferential, and still may be deemed satisfactory. Distinct fossils have not been found, all that ever existed in the azoic[90] rocks having been obliterated. The arguments in the affirmative are as follows: "1. The existence of limestone rocks among the other beds, similar limestones in later ages having been of organic origin; also the occurrence of carbon in the shape of graphite, graphite being, in known cases in rocks, a result of the alteration of the carbon of plants. "2. The fact that the cooling earth would have been fitted for vegetable life for a long age before animals could have existed; the principle being exemplified everywhere that the earth was occupied at each period with the highest kinds of life the conditions allowed.