PHYSICAL CONDITIONS IN THE OCEAN 



159 



temperature. Notable among these is the increased number of vertebrae 

 in teleostean fishes from south to north and from warm waters to 

 cold. 29 Among acanthopterygian fishes the vertebrae number uni- 

 formly 24 in all the tropical species of a whole series of families, 

 while the cool-water species belonging to these same families have an 

 increased number. Among flounders of the subfamily Hippoglossinae, 

 the number of vertebrae increases from 35 in the more southern species 

 on the American coasts to 50 in the Arctic. 30 Increase in length in 

 m 13! J?! 



500 



1000 



1500 



9°. 



Fig. 14. — Diagrammatic vertical section across the Wyville-Thomson Ridge with 



the temperature stratification shown. 



arctic fishes is produced by increase in the number of vertebrae, while 

 many elongate fishes of the tropics have acquired the same body form 

 by increase in the length of the individual vertebrae, without increase 

 in their number. 



The larger size of northern and cold water forms is a frequent 

 phenomenon, both as applying to the individuals of a wide-ranging 

 species, 31 " 41 and to the northern or cold-water species which replace 

 tropical forms within a given genus. 4858 Delayed sexual maturity, 

 which favors growth in size, appears to be directly dependent on 

 retardation of growth at lower temperatures, and this is a common 

 characteristic of northern marine creatures. 42 " 47 



Large-yolked eggs, with more complete development of the embryo 

 within the egg, and the brood care or ovoviviparity which are asso- 



