80 ORIGIN OF CHORDATES in. 10 



Can we see in the production of the first fish-like creatures clear 

 signs of an 'advance' in evolution ? In acquiring the power of active 

 muscular locomotion the animals became able to live and feed in a 

 variety of habitats, either at the sea surface or on the bottom. Forms 

 with a sedentary adult stage are limited by the necessity for the 

 presence of a sea bottom of suitable character. The larvae were evolved 

 to provide the information to make sure of reaching such conditions. 

 But whereas suitable situations on the bottom are not common, and 

 are liable to change, the sea surface provides a generalized habitat in 

 which there is always abundant food, though no doubt also strenuous 

 competition for it. Paedomorphosis in this case, as in others, allows 

 the race to eliminate from its life-history the stage passed in a 'special' 

 environment, which is difficult to find. Although the fish-form that 

 was thus produced proved to have great possibilities for further 

 evolution, the change was not at first a strikingly progressive one. 

 The surface of the sea is perhaps the most general of all environments ; 

 possibly it was the seat of the origin of life. Races that have devised 

 means of living on the sea bottom may therefore be said to have 

 advanced, because they have invaded a more difficult habitat. To 

 abandon the sedentary life might in this sense be regarded as a retro- 

 grade step. The peculiar feature of the early fishes, however, was that 

 they developed powers of active movement in a relatively large 

 organism provided with efficient receptors, and by making use of the 

 feeding mechanism developed at first by the bottom-living adult were 

 able to live successfully at the sea surface. They acquired their 

 dominance at this stage not by invading new habitats but by develop- 

 ing effective means of living in the richly populated plankton. 



