If all plants were removed from the earth, animals could not exist. How would it be if the animal kingdom was annihilated?--would it be possible for vegetation to continue? This question is not quite so easily answered; but, if we suppose all the carbon-producing machines--the animals--to be extinct, from whence would the plants draw their supply? It has been supposed that during the epoch of the coal formation a luxuriant vegetation must have gone on over the earth’s surface, when the existence of animal life was regarded as problematical. It is supposed that the air was then charged with carbonic acid, and that the calamites, lepidodendra, and sigilaria, were employed to remove it, and fit the earth for the oxygen-breathing races. The evidence upon these points is by no means satisfactory; and although at one time quite disposed to acquiesce in a conjecture which appears to account so beautifully for the observed geological phenomena of carboniferous periods, we do not regard the necessities for such a condition of the atmosphere as clearly made out.[223] Geological research, too, has shown that the immense forests from which our coal is formed teemed with life. A frog as large as an ox existed in the swamps, and the existence of insects proves the high order of organic creation at this epoch. In all probability the same mutual dependence which now exists between the animal and vegetable kingdoms existed from the beginning of time, and will continue to do so under varying circumstances through the countless ages of the earth’s duration. There is yet another very important chain of circumstances which binds these two great kingdoms together. This is the chain of the animal necessities. A large number of races feed directly upon vegetables; herbs and fruits are the only things from which they gain those elements required to restore the waste of their systems.