156 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



come of purely pelagic influence. The wanderers from the bottom have 

 introduced another factor in the evolution of pelagic life, for their bodies 

 have been utilized for purposes of protection or concealment or on 

 account of other advantages, and we now have fishes which shelter 

 themselves in the poisoned curtain of physalia ; Crustacea which live in 

 the pharynx of salpa; barnacles and sucking fishes fastened to whales 

 and turtles, besides a host of external and internal parasites. The primi- 

 tive ocean furnished no such opportunity, and the conditions of pelagic 

 life must, at first, have been extremely simple. 



Among the higher metazoa and the higher plants size is, in itself, an 

 important factor in evolution. Variations in the constituent cells of a 

 large organism are continually 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 vegetable 

 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 Ife 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 speculation, 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 



