D.— ZOOLOGY. 



93 



potent as a source of fat-soluble vitamin. \Vc thus conclude that this 

 substanco, so essential to healtliy animal growth, is iiroduced in large 

 quantities by plankton diatoms, and passed on unchanged to the fish 

 through the crustaceans which feed on the diatoms. In the fish the 

 vitamin is first stored in the liver, and with the ripening of the ovary 

 passes into the egg, to be used to stimulate the growth of the next 

 generation. Again we see the fundamental importance of the food- 

 producing activities of the lowest plant life. 



Attention has already been drawn to the suggestion that fishes 

 developed their remarkable swimming powers in rivers, in response to a 

 need to overcome the' currents, and that they afterwards returned to 

 the sea, where they preyed upon a well-developed and highly complex 

 invertebrate fauna already fully established there. Their speed enabled 

 them to conquer their more sluggish predecessors, whilst they them- 

 selves were little open to attack. With the exception of the larger 

 cephalopods, which are of comparatively recent origin, and were 

 probably evolved after the arrival of the fishes, there are few, if any, 

 invertebrates which capture adult fishes as part of their normal food. 

 Destinictive enemies appeared later in the form of whales and seals 

 and sea-birds, which had developed on the land and in the air. 



And now in these last days a new attack is made on the fishes of 

 the sea, for man has entered into the struggle. He came first with 

 a spear in his hand; then, sitting on a rock, he dangled a baited hook, 

 a hook perhaps made from a twig of thorn bush, such as is used to 

 this day in villages on our own east coast. Afterwards, greatly daring, 

 he sat astride a log, with his legs paddled further from the shore, and 

 got more fish. He made nets and surrounded the shoals. Were there 

 time we might trace step by step the evolution of the art of fishing and 

 of the art of seamanship, for the two were bound up together till the 

 day when the trawlers and drifters kept the seas for the battle fleet. 



There can be little doubt that in European seas the attack on the 

 fishes in the narrow strip of coastal water where they congregate has 

 become serious. A considerable proportion of the fish population is 

 removed each year, and human activity contributes little or nothing 

 to compensate "the loss. We have not, however, to fear the practical 

 extinction of any species of fish, the kind oi extinction that has taken 

 place with seals and whales. Fishing is subject to many natural 

 limitations, and when fishing is suspended recovery will be rapid. There 

 is evidence that such recovery took place in the North Sea when fishing 

 was restricted by the War, though the increase which was noted is 

 perhaps not certainly outside the range of natural fluctuations. Until 

 the natural fluctuations in fish population are adequately understood, 

 their limits determined, and the causes which give rise to them dis- 

 covered, a reliable verdict as to the effect of fishing is difficult to obtain. 



If such problems as these are to be solved the investigation of the 

 sea must proceed on broadly conceived lines, and a comprehensive 

 knowledge must be built up. not only of the natural histoiy of the 

 fishes, but also of the many and varied conditions which influence their 

 lives. The life of the sea must be studied as a whole. 



