45 2 



NA TURE 



{S 



EPTEMBER 30, I 92: 



could produce was then reaching its summit and be- 

 coming stabilised. Since Cambrian times geologists 

 tell us some thirty million years u have passed, a stretch 

 of tii):e which it is really difficult for our imaginations 

 to picture. During that time a change of immense 

 moment has happened to the life of the sea ; but if 

 id the signs aright, that change had its origin 

 rather in an invasion from without than in an evolution 

 from within. From whence came that tribe of fishes 

 which now dominates the fauna of the sea ? It would 

 Lie rash to say that we can give any but a speculative 

 reply to the question, but the probable answer seems 

 to be that fishes were first evolved not to meet conditions 

 found in the sea. but to battle with the swift currents 

 of rivers, where fishes almost alone of moving animals 

 can to this day maintain themselves and avoid being 

 swept helplessly away, 12 It was in response to these 

 conditions that elongate, soft-bodied creatures, which 

 had penetrated to the river mouth, developed the 

 slender, stream-lined shape, the rigid yet flexible 

 musi ular body, the special provision for the supply of 

 oxygen to the blood to maintain an abundant stock 

 of energy, and all those minute perfections for effective 

 swimming that a fish's body shows. The fact that 

 many sea fishes still return to the rivers, especially for 

 spawning, supports this view, and it is in accordance 

 with Traquair's classical discoveries of the early fishes 

 of the Scottish Old Red Sandstone, which were for the 

 most part fresh- and brackish-water kinds. 



Having developed, under the fierce conditions of 

 the river, their speed and strength as swimmers, the 

 li she returned to the sea, where their new-found 

 powers enabled them to roam over wide areas in search 

 of loud, and gave them such an advantage in attack 

 and defence that they became the predominant in- 

 habitants of all the coastal waters, and as such they 

 remain to-da] . 



The other great migration of the fishes, also, the 

 migration from the water to the land, giving rise to 

 amphibians, reptiles, birds and mammals, must not 

 be left out of account. The whales, seals, and sea-birds 

 which, after developing on land, returned again to the 

 waters and became readapted for life in them, are 

 features which cannot be neglected; 



And so we are brought to the picture of life in the sea 

 as we find it to-day. The primary production of organic 

 substance by the utilisation of the energy of sunlight in 

 the bodies of minute unicellular plants, floating freely in 

 the water, remains, as it was in the earliest times, the 

 feature of fundamental importance. The conditions 

 whii h control this production are now. many of them, 

 known. Those of chief importance are (1 ) the amount of 

 light which enters the water, an amount which varies 

 with the length of the day, the altitude of the sun, and 

 the clearness of the air and of the water : (2) the presence 

 in adequate quantity of mineral food substances, 

 especially nitrates and phosphates ; and (3) a tem- 

 perature favourable to the growth of the species which 

 are present in the water at the time. Experiments 

 with cultures of diatoms have shown clearly that if 

 the food-salts required are present, and the conditions 

 as to light and temperature are satisfactory, other 



ctoi . such as the salinity of the water and the pro- 



11 1 Evolution of Lift 



1 I oamberlin, 'Rioted in Lull, "Organic Evolution," New \ 



NO. 2761, VOL. I IO] 



portions of it- constituent salts, can be varied within 

 very wide limits without checking growth. The 

 increased abundance of plankton, especially of diatom 

 and peridinian plankton, in coastal waters and in 

 shallow seas largely surrounded by land, such as the 

 North Sea. is due to the supply of nutrient salts washed 

 directly from the land by rain or brought down by 

 rivers. An exceptional abundance of plankton in 

 particular localities, which produces an exceptional 

 abundance of all animal life, is also often found where 

 there is an upwelling of water from the bottom layers 

 of the sea. These conditions are met with where a 

 strong current strikes a submerged bank, or where 

 two currents meet. Food-salts which had accumulated 

 in the depths, where they could not be used owing to 

 lack of light, are brought by the upwelling water 

 to the surface and become available for plant growth. 

 The remarkable richness of fish life in such places as 

 the banks of Newfoundland and the Agulhas Banks 

 off the South African coast . each of which is the meeting- 

 place of tw"0 great currents, is to be explained in this 

 way. 



Attention has already been directed to the suggestion 

 that fishes developed their remarkable swimming 

 powers in rivers in response to a need to overcome the 

 currents, and that they returned afterwards to th 1 

 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, while they themselves were 

 little open to attack. \\ ith the exception of the larger 

 cephalopods. which are of comparatively recent origin, 

 and were probably evolved after the arrival of the 

 fishes, there are lew. if any. invertebrates which capture 

 adult fishes as part of their normal food. Destructive 

 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, lie 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 clay when the trawlers and 

 drifters kept the seas for the battle fleet. 



There can lie little doubt that in European seas the 

 attack on the fishes in the narrow strip of coastal water 

 where thev congregate has become serious. A con- 

 siderable 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 of extinction that has taken place with seals 

 and whales. Fishing is subject to many natural 

 limitations, and when fishing is suspended recover)' 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 



