52 CHILDHOOD OF ANIMALS 



them, as in reptiles, although there are species which may reach a 

 large size and species the members of which are always small, 

 there is a very wide range of size for each species, and growth 

 appears to go on continuously throughout life. As in the case of 

 reptiles, the rate of growth varies with external conditions, partly 

 those of temperature, but still more the nature and amount of the 

 food-supply. In many fish there are annual ring-like markings on 

 the scales, and in others in the concretions found in the internal 

 ear, and known as otoliths, by which the age can be estimated, in 

 the same way as the age of a tree can be ascertained by counting 

 the annual rings of growth visible when the stem is cut across. 

 Experiments made by transferring marked fish from places where 

 the food-supply is scanty to places where it is abundant have shown 

 that the size of a fish cannot be taken as an indication of its age. 

 The eggs of fish take from about three to over a hundred days to 

 hatch out, but the time varies a good deal according to the tempera- 

 ture of the water. As a rule the eggs of smaller fish hatch more 

 quickly than those of larger fish, but a more important difference 

 depends on the size of the egg. Small eggs with very little yolk 

 hatch quickly, and the larvae on their appearance are in a more 

 rudimentary condition. Those with an abundant supply of yolk 

 take longer to hatch, but the larvae are relatively larger and more 

 highly developed. As cold water delays development and retards 

 the period of hatching, the larvae usually appear when the water is 

 warm and when there is an abundant supply of the microscopic 

 organisms on which they feed. Growth is then rapid and in most 

 cases the larvae become transformed into small fish like the adults 

 in the course of their first season. The subsequent history varies 

 much in different kinds of fish. In those where the larvae and the 

 adults live under practically the same conditions, the sexual organs 

 often mature next season, and although the fishes may be small, 

 their period of youth is over. Often there is a migration from 

 inshore water to deep water, to the bottom of the sea, or, in the 

 case of fresh water, from the shallow fringes of lakes or from upland 

 streamlets to deep water or to the lower parts of rivers, and the 

 change to adult life may take more than a season. In fishes where 

 there is a complete change of habitat the youth may be further 

 prolonged. The larvae of the salmon, called parr or samlets, are 

 hatched in the spring in the fresh-water pools where the spawn has 

 been deposited. They remain in the rivers usually for about two 

 years, slowly losing their youthful uniform of red spots and dark 



