104 



CHAPTERS ON EVOLUTION. 



in symmetrical array. And we also discover that in most fishes with 

 apparently equal tails, the spine is really unsymmetrical (Fig. 42), and 

 projects into the upper half a condition of affairs more visibly 

 exemplified in certain fishes of which the sharks (Fig. 41) and dog- 

 fishes are the best known examples. 



The inferences and conclusions regarding the general development 

 of the fish-class which may be deducible from the brief consideration 

 of the structure of the tail in these animals, will be apparent if we 

 venture to trace the development of the tail, and to take a wide survey 

 of the succession of the fishes in time, as represented in the records 

 of the rocks. It is perfectly clear, to begin with, that the tails of all 

 fishes are modelled upon one and the same type. In so far as the 

 prevalence of one modification of a type over another may be said 

 to indicate the primary form of the type, we may hold that the fish- 

 tail begins as a straight appendage, to which must be assigned the place 

 of honour, as probably the first, most primitive, and least modified 

 form of the fish-tail, whilst to this straight condition succeeds the 



unequal variety. And the story told 

 by development very plainly endorses 

 this statement. All fish-tails, what- 

 ever their ultimate and adult form, 

 exhibit the unequal type as their 

 second well-defined condition. The 

 equal appearance of the tail of the 

 vast majority of fishes is thus proved 

 beyond doubt to be a third and com- 

 paratively modern innovation, so to 

 speak, in the matter of fish history. 

 If we appeal to the researches ot 

 Alexander Agassiz on this point, we 

 may learn much that is instructive and 

 edifying on this curious question. In 

 the development of the flounder, for 

 instance, the soft rod, named the noto- 

 chord, which at first does duty for the 

 spine, is seen to possess a straight 

 extremity (Fig. 43, A s) ; and the 

 embryonic or first-formed tail-fin is 

 simply rounded in shape. Very 

 soon the tail end of the notochord 

 becomes bent upwards, and carries up with it in its progress the primi- 

 tive tail-fin. At this stage (Fig. 43, B) the permanent tail-fin becomes 

 marked out and defined from the primitive fin, and in due course, and 

 even before the spine itself has become developed, the young flounder 

 possesses an unequal tail. In this stage the young flounder, indeed, 



FIG. 43. 

 DEVELOPMENT OF THE TAIL IN FLOUMDER. 



