STRUCTURE OF THE VERTEBRATES 47 



Evolution of the Fish. The student should keep in mind that 

 no living group of animals can be the direct ancestors of an- 

 other. From this standpoint '4iigher" and ''lower" are rather 

 meaningless. The terms are based on the human concept of 

 superiority, and to a large extent refer to the evolution of the 

 nervous system; although we may say that '^higher" animals are 

 those which have evolved farthest from the fundamental, primi- 

 tive characteristics. 



We may also logically speak of the fish ancestors of men, not 

 meaning any living group, but some common ancestral type 

 which evolved in several directions. One group of fish became 

 better adapted for certain aquatic conditions, another for land 

 conditions; or, stated differently, a group which was generalized 

 had a far better chance of developing in several directions than 

 did a more specialized type. 



All the above conditions are abundantly illustrated by the 

 fish. The earliest ancestor was an amphioxus-like chordate. From 

 this animal numerous varieties developed, the recent Amphioxi 

 having remained most nearly like the ancestral form. Another 

 variation from the original ancestor became more cyclostome in 

 structure, and from this group new variations appeared. Those 

 which most resemble the ancient stem form are the Cyclostomes, 

 while other groups which added new^ characters enabling them 

 to cope better with environmental conditions evolved into primi- 

 tive fish. 



All the earliest known fossil fish belong to the elasmobranchs, 

 and there again we see evolution proceeding in many directions. 

 The generalized sharks were not only capable of surviving in 

 more varied environments, but were more liable to develop new 

 mutations having a survival value. 



Some of the adaptations have been mentioned. Those which 

 tended toward respiration and locomotion on land, as did the 

 specializations of the Crossopterygii, naturally opened up an 

 enormous area as a habitation for the vertebrates. They were 

 poor fish but excellent ancestors; and therefore Darwin's '^sur- 

 vival of the fittest" has been modified to '^survival of the best 

 adapted for a particular environment". 



