40 EVOLUTION IN THE PAST 



of primitive character. The " backbone " composed of 

 cartilage was unjointed ; but, not being stoutly developed, 

 it retained a fair amount of flexibility. The paired fins seem 

 to have been capable only of an up-and-down flapping move- 

 ment ; and these, though useful as balancers, can have been 

 but of little service for guiding purposes, or for assisting in 

 onward locomotion. Their character gives support to the 

 theory that fins originated as mere folds of flesh, strengthened 

 by outgrowths from the internal skeleton ; and that in time 

 the folds became divided up into distinct flippers, which 

 gradually developed increased functional powers. The tail, 

 however, is the great organ of propulsion ; and these fishes 

 were not devoid of strength in that quarter. 



More advanced in development were certain small sharks, 

 forerunners of which had appeared in the Silurian (Climatius, 

 Mesacanthus). The fins of these fishes, both vertical and 

 paired, were well developed ; and, excepting the tail-fin, 

 were fore-guarded by spines. Some of these little creatures 

 were as much as a foot in length, and no doubt were the 

 elite of shark-life at this time. 



CHIMERAS Other shark-like forms, to judge by their teeth, were 

 developing in the direction of chimaeras (Ptyctodus). But 

 the goal to which they were tending was as yet a long way off. 

 GANOIDS Other fishes of more engaging aspect were moving about 

 in bony scales enamelled with a glistening substance (ganoin). 

 This shining habiliment was doubtless the result of modifica- 

 tions of small tooth-like plates such as were formed in the 

 skin of early shark-like forms : but how the transformation 

 was effected is quite unknown. 



CROSSOP- Most of the ganoids as these fishes are called possessed 



TERYGIANS somewhat stiff, paddle-like fins ; and as the appendages were 

 bordered with thread-like structures, their owners are known 

 as " fringe-finned " ganoids (Crossopterygii). The median 

 fins useful in keeping a fish straight on its course had not 

 supports equal to the number of the rays, as is the case with 

 modern fishes. And the tail was not of the type best adapted 

 for swift straight-ahead swimming. All these features 

 tend to show that these ganoids cannot have been accom- 

 plished swimmers, at least according to modern ideas. 



