CREATION BY EVOLUTION
of the Devonian coal forests. Man also owes to these Devonian fishes his very ability to breathe.
Man has, moreover, inherited from these amazingly ancient animals each and every one of his twenty-eight skull bones, as well as every other bone of his entire skeleton. But the earlier types possessed many more bones in the skull than man does. The late Professor Williston, of the University of Chicago, showed that as we pass from earlier to later vertebrates reduction in the number of the skull elements is the rule, although there is here and there an exception due to the fragmentation of one of the remaining elements.
Fishes, in common with all other vertebrates, have, on each side of the head, the three semicircular canals which act like spirit levels and assist the animal in maintaining its equilibrium. Below the semicircular canals is a sack supplied with nerves. A part of this sack corresponds to the true organ of hearing in man. The cavity of the human ear (behind the drum membrane) is represented in fishes by the cavity of the gill chamber. Fishes have no drum membrane, which first appears in the amphibians, but its place is occupied in the fishes by the bony shell or operculum covering the outer side of the gill chamber. When the air-breathing fishes were changed into amphibians the opercular series disappeared, leaving only a notch at the outer upper corner of the skull, upon which was stretched the drum membrane of the ear.
Thus, by the time amphibians originated from some air-breathing lobe-finned fishes, all the fundamental problems in the production of the ground plan of the human organization had been solved, and it is literally true that the oldest amphibians were on the whole much nearer to man than they were to the oldest known ostracoderms. Nevertheless, a host of improvements in the entire mechanism were still to be worked out, and to these striking advances we now turn
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