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CHAPTERS ON EVOLUTION. 



FIG. 45. --LAMPREY AND ITS BREATHING APPARATUS. 



history. Let us inquire, for instance, regarding the nature of the 

 oldest fishes, and trace the piscine race downwards to existing 

 times, as completely as the scattered pages of nature's records will 

 admit. If we discover that the succession of fish-tails in time cor- 

 responds with the order of their development to-day -, we may then be 

 certain that the history of the individual repeats the history of its race. 

 Of the very lowest fishes, it must firstly be remarked, we possess 

 fro traces or record in a fossil state. These democrats of the fish 



class are represented by 

 the existing lancelet (Fig. 

 44), a tiny fish about an 

 inch and a half long, with 

 a soft and perfectly trans- 

 parent body ; and by the 

 lampreys (Fig. 45) and 

 hag-fishes the latter 

 found boring their way 

 into the bodies of cod 

 and other fishes by means 

 of a single large tooth 

 borne on the palate. The 

 lancelet, lampreys, and 

 hag-fishes possess no hard parts which could have been preserved 

 in a fossil condition. Yet, from all considerations regarding their 

 lowness of structure, we are forced to conclude that these fishes 

 possess an immense antiquity, and probably represent the primitive 

 founders of the entire fish class. 



The first rocks in which the fossil remains of fishes occur are the 

 Upper Silurian strata. These first traces at once of fish and verte- 

 brate life consist of the fin-spines, &c, of fishes evidently allied to 

 our existing sharks fishes which possess, as we have seen, the 

 primitive type of the unequal tail. Throughout the succeeding ages 

 of the Palaeozoic period a period including in its later epochs 

 the Old Red Sandstone, Coal, and Permian formations the type of 

 fish-tail remains practically unaltered, and presents us with the un- 

 equal form. Nowhere is the unequal tail more typically seen than 

 in the famous fishes of the Old Red Sandstone (Fig. 46), which, clad 

 in a stout shield-like armour of " ganoid " scales, like our living 

 bony pike (Leptdosteus} and sturgeon, must have presented well- 

 nigh impregnable fronts to their adversaries. As Owen remarks, 

 "the preponderance of heterocercal (unequal-tailed) fishes in the 

 seas of the geological epochs of our planet is very remarkable ; 

 the prolongation of the superior lobe (or upper half) characterises 

 every fossil fish of the strata anterior to and including the Magnesian 

 Limestone (Permian rocks); the homocercal (even-tailed) fishes first 



