GENERAL BIOLOGY 765 



The study of numerous samples taken from the fish-stock Age-com- 

 of a certain area may aim at ascertaining the age -com- tSock^of 

 position of that stock, and from the results the follow- fishes. 

 ing main points in the natural history of the fish may be 

 elucidated: (i) the age at which maturity occurs; (2) the 



A B 



Fig. 560. 



Two scales of fi\'e-year-old herrings. A, growth under normal conditions ; B, abnormal growth 



in the third zone. 



duration of life ; and {3) the variations in the age-composition 

 and magnitude of the fish-stock. 



Studies of this kind have shown us that various species 



A B 



Fig. 561. 



Scales of .4, herring [Clupea hareng/is) ; B, sprat {Cliipea sprattus). Both fishes i6 cm. long. 



are distinct even in this respect. Nothing shows this 

 more clearly than a comparison between the two closely 

 related species : the sprat and the herring. Fig. 561 represents 

 scales of a herring and of a sprat, both 16 cm. long, the 

 herring being only \\ year old and the sprat 4 years. The 



