500 THE AMERICAN NATURALIST [Vol. LVI 



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 muscu- 

 lar 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, sup- 

 ports this view, and it is in accordance with Traquair's 

 classical discoveries of the early fishes of the Scottish 

 Old Bed 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 en- 

 abled them to roam over wide areas in search of food, 

 and gave them such an advantage in attack and defense 

 that they became the predominant inhabitants of all the 

 coastal waters, and as such they remain to-day. 



The other great migration of the fishes, also, the migra- 

 tion from the water to the land, giving rise to amphibians, 

 reptiles, birds and mammals, must not be left out of ac- 

 count. The whales, seals and sea-birds, which after de- 

 veloping on land returned again to the waters and became 

 readapted for life in them, are features which can not 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 utilization 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 importance are (1) the amount of light which 



21 Chamberlin, quoted in Lull, "Organic Evolution/' New York^ 1917, 

 p. 462. 



