A FISH FROM A PELAGIC EGG. 65 



On the other hand, the witch {Pleuronectes cynoglossus) 

 keeps to its special areas, both as regards the young and the 

 adult condition, so that the movements of eggs, larval and 

 post-larval forms are circumscribed ; and the same, to some 

 extent, would seem to be the case with the topknot (Zeugo- 

 pterus) and sail-fluke (Arnoglossus). The dab {Pleuronectes 

 limanda), again, is found in all stages both in comparatively 

 deep and in comparatively shallow water. 



Almost all our valuable food-fishes, therefore, are produced 

 from minute pelagic eggs, the enormous numbers of which 

 provide for a vast increase and wide distribution of the species ; 

 yet it cannot be said that this habit alone provides for their 

 multiplication when the case of the herring with its demersal 

 eggs, fixed firmly to the bottom, is considered. It has to be 

 borne in mind, however, that the larval herring mounts upward 

 toward the surface as soon as its strength suffices. 



Many striking changes occur during growth, both in external 

 form and coloration, but it is difficult at present to lay down 

 any general law that would apply to all cases, though those in 

 which certain migrations take place during growth show such 

 changes very prominently. The young round fishes by-and-by 

 roam about the sea in shoals, led hither and thither mainly by 

 the presence of food; yet in the case of the larger and adult 

 forms, safety or freedom from molestation may have some 

 influence. Though so minute on escaping from the egg, their 

 growth is, by-and-by, rapid, and the duration of life in such as 

 the cod is considerable. Abundance of food, more than any 

 special instinct, would appear to be the main cause of their 

 migrations in the adult or adolescent state, and that food is 

 as varied as their haunts; in short it embraces every sub- 

 kingdom up to their own, for fishes and their eggs form a 

 large share of their diet. 



There would be little difficulty in adding to the sea great 

 numbers of larval forms of any species of which eggs can be 

 procured : yet if a few adults can be obtained in such waters 

 at the proper season it is still an open question whether the 

 natural process with its surroundings would not be more 

 successful. 



