152 THE SHAPES AND SIZES OF ANIMALS 



mere increase of size may involve and necessitate 

 greater complexity of structure. 



Amongst Vertebrates we have already noticed 

 the extreme individual variability of size seen 

 amongst fish, in which apparently growth continues 

 throughout the whole period of life. The actual 

 limits of size of fish are probably only imperfectly 

 known to us. The blue shark, Carcharias, attains 

 a length of twenty-five feet ; specimens of Car- 

 charadon have been measured over forty feet in 

 length, while of the genus Rhinodon examples of 

 fifty, sixty, or even seventy feet in length have 

 been described. This is very probably the limit 

 of size reached by fish at the present day, but 

 judging from the fossil teeth of Carcharodon, 

 Cestracion, and other forms, sharks of these genera 

 must have existed in tertiary times more than 

 twice the dimensions of any now living. 



It is difficult to speak with any certainty about 

 the size of Amphibians, but the existing genera 

 are all small. Some of the Salamanders attain a 

 length of four feet or more, but the majority of 

 recent Amphibians are of much smaller size. 

 Fossil forms occur of much larger dimensions, 

 some, such as the Labyrinthodonts, being veritable 

 giants. It is however impossible in most cases 

 to speak with certainty as to the Amphibian char- 

 acter of these fossils, but we are probably right 

 in regarding recent Amphibians as diminutive and 

 pigmy representatives of a group formerly of much 

 larger size. 



Reptiles are in much the same position, many 



