D.— ZOOLOGY. 91 



notice here. From the molhiscs, creeping on soHd surfaces, sprang 

 a group of swimmers, the Cephalopods, which have grown to sizes 

 almost unequalled amongst the animals of the sea. 



All these invertebrate phyla had become established and most of 

 them had reached a high degree of development in the seas of Cambrian 

 times. Amongst animals then living there are many which have 

 survived with little change of form until to-day. One is almost 

 tempted to suggest that the life which the sea itself could produce was 

 then reaching its summit and becoming stabilised. Since Cambrian 

 times geologists tell us some thirty million years'^ have passed, a stretch 

 of time which it is really difficult for our imaginations tO' picture. 

 During that time a change of immense nioinient has happened to the 

 life of the sea ; but if we read 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 be rash to say that we can give any but a specu- 

 lative 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.-" 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 muscular 

 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 fishes returned to the sea, where 

 their new-found powers enabled them to roam over wide areas in search 

 of food, and gave them such an advantage in attack and defence that 

 they became the predominant inhabitants of all the coastal waters, 

 and as such they remain to-da-y. 



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 which control 

 this production are now, many of them, known. Those of chief import- 



" Osborn, Origin and Eiwlufion of Life, 1918, p. 153. 



'" Chamberlin. quoted in Lull, Organic Evohition, New York, 1917, p. 462. 



