THE PROGRESSION OF LIFE ON EARTH
same time the scales became thinner and deeply overlapping and contained very little bone substance and enamel.
In the following Cretaceous seas there were a few fishes that had the bony support of the tail as well formed as that of most existing bony fishes; indeed, the reign of the modern thin-scaled bony fishes, completely adapted for rapid movement in water, had begun, and the only subsequent changes were those which have given almost endless variety to this thoroughly efficient race. It is also interesting to note that the fishes which achieved these latest developments include nearly all those that are used as food by man today.
To summarise briefly: The first fishes were encumbered with outside armour and their fins were not very well formed for swimming. Next, the paired fins became thoroughly adapted for balancing, and then the tail fin was improved until it became a perfect propeller. After this the inside skeleton became bony and gradually grew more efficient and more complicated, and the scales of many forms became thin. Thus, by progressive stages, in a definite order, fishes were continually improved for locomotion and for feeding in water, and there arose the possibility of the infinite variety found among the existing bony fishes.
A student of fossils recognises that when any kind of animal shows a tendency to change in some particular part, the degree of this change increases in successive generations, especially if the change at first gives it some advantage. Among the later ganoid fishes of the Jurassic period there are some that tend to assume the form of a swordfish, which has a powerful tail that fits it for darting as well as for swimming. The snout begins to thrust itself forward at the front of the upper jaw. Toward the later part of the Jurassic period the snout even forms a pointed weapon. In the middle part of the Cretaceous period, which followed,
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