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 move- 

 ment 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 ani- 

 mal 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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