CREATION BY EVOLUTION
from the Greek work for resplendent. Their arm fins and leg fins were stiff paddles, used more for crawling on the bottom than for propulsion or balancing in swimming. Some that had powerful jaws, such as Dinchthys (“terrible fish’’), became gigantic, having heads three or four feet across and armour in places three or four inches thick. Like our early steam battleships, they specialised in weight of armour, and like these battleships they were soon superseded by rivals which depended for success on swift movement rather than on stolid defence. In a few of the Devonian ganoids the two pairs of paddles were replaced by ordinary flexible fish fins, which were strengthened by fin rays, and in some the paired fins were halfway between these two patterns.
During the next period (the Carboniferous) ganoids with flexible paired fins predominated, but they were still handicapped as swimmers by the low degree of hardening of the internal bones, by the incompleteness of the tail as a swimming apparatus, and by the unfinished mechanism of the fins along the middle of the body above and below. The tail was formed by the tapering end of the body, turned upward to make an upper lobe; the real fin was below this, as in the sharks and sturgeons of the present day.
During the Permian and Triassic periods, which followed, the tail in the more progressive fishes lost its upper body lobe by shrinkage and became a most efficient tail fin, and the middle fins were gradually brought up to the most efficient form. The internal skeleton was also gradually hardened.
Early in the next period (the Jurassic) the backbone in some fishes was completed. Each joint or vertebra was deeply hollowed at each end to admit soft, elastic substance and so to give the great flexibility that is needed for rapid swimming. The bony skull was also completed. At the
[ 130 ]
