SALPA IN RELATION TO EVOLUTION OF LIFE. 147 



stituent cells of a large organism are coatinually being seized upon 

 and fixed by natural selection, on account of their value in the 

 functions of relation to other parts. Primitive pelagic organisms 

 are all minute, and it is easy to understand why. To plants which 

 are bathed on all sides by food, like the pelagic protophytes, small 

 size is advantageous, since a small body has a larger surface in 

 proportion to its bulk than a large one ; and the pelagic plants 

 are, as I have shown, most favorably placed for rapid growth when 

 new cells separate as soon as they are formed, and thus expose all 

 their surface. 



The same ratio between bulk and nutritive surface tends to limit 

 in the same way, if not to the same degree, the growth of the pelagic 

 animals which live in the midst of an abundant supply of vegeta- 

 ble food. 



Competition was not entirely absent among the primitive pelagic 

 organisms, for the conditions of life are never absolutely uniform, 

 although the possibilities of evolution must have been extremely 

 limited and the progress of divergent modification very slow, so 

 long as life was restricted to the waters of the ocean. 



There can be no doubt that pelagic life was abundant for a long 

 period during which the bottom was uninhabited. The history of 

 the slow process of geological change by which the earth gradually 

 assumed its present character, presents a boundless field for specu- 

 lation, but there can be no doubt that the surface of the primeval 

 ocean became fit for life long before the deeper waters or the sea- 

 floor. 



The early steps in the evolution of plants must have been taken 

 in the transparent surface water under the influence of sunlight, 

 and as both animals and plants are dependent upon oxygen, the 

 primal flora and fauna must have lived in aerated water. The 

 oxygen which is diffused through the ocean from the surface, 

 where it is absorbed from the air, is gradually exhausted by 

 oxidizable substances, both inorganic and organic, and it dimin- 

 ishes with the distance from the source of supply at the surface. 

 The oceanic circulation tends to equalize its distribution, and no 

 part of the ocean now seems to be totally without oxygen. Oxygen 

 has been shown to be reduced to a minimum at the bottom of some 

 of the great depressions of the sea-floor, and it is clear that a slight 



