THE LIFE-HISTORIES OF FISHES 167 



from side to side, and the head begins to twist, so 

 that by and by both eyes appear to lie on one 

 side of the body. Sooner or later the fish passes 

 out of its larval into its " post-larval " stage, and 

 can be recognised easily as belonging to a definite 

 species. Hitherto it has been drifting helplessly 

 about at the surface, or indeed at any depth in the 

 sea ; but with the termination of the larval or post- 

 larval stage it acquires the habits and enters the 

 habitat of the adult. The assumption of sexual 

 maturity may be long delayed, but after a period 

 of from one to three or four years, depending on 

 the species, the reproductive organs become mature 

 and the fish begins to produce ova or spermatozoa, 

 according to its sex. 



When we consider the enormous differences in 

 the fecundity of various fishes, we encounter a diffi- 

 culty which at first sight is not easily explained. 

 A turbot, we have seen, may spawn each year some 

 nine millions of eggs, each of which is potentially 

 a distinct fish ; on the other hand, a skate lays two 

 eggs at a time, and probably only produces a dozen 

 or so during its hatching season. Yet common ex- 

 perience shows that skate are much more numerous 

 in our seas than turbot, in spite of this almost 

 incredible difference in fecundity. Every species 

 of organism in the sea is subject to enormous de- 

 struction. There can be no doubt that the pelagic 

 eggs of the turbot are devoured by numerous pre- 

 daceous animals, as are also the larval and the post- 



