44 



brick pavercsnt. Related to this necessity of reducing to a miniraiKi 

 the loss of inoieture through the surface, is the equal necessity 

 that almost every land animal or plant is under, of replacing such 

 losses 'jy taking in water; i.e., the need of drinking water. It is 

 a commonplace of school boy instruction that the supply of drink- 

 ing water, and the dampness of the air, together, go far to control 

 the relative fertility of different parts of the lands, and the 

 kinds of animals and plants that can inhabit them. 



The very facts noted above, however, enable the land-organism 

 more, readily to perfect a "milieu interieur" adapted to specific 

 purposes. Thus the evolutionary attempt, exerted on land, to 

 escape this control by the water supply, has resulted in a grap.t 

 variety of protective adaptations, of which the water metabolism 

 of desert insects, and the storage of water by the stomach of a 

 camel, or in the stems and loaves of certain desert plants, are 

 perhaps the most familiar examples. In fact, it is no exaggeration 

 to say that the life of every aniraal or plant on land is directed 

 by the need of metabolic water. Even the briefest failure in the 

 supply is apt to be fatal. 



From all this water-complex all the animals and plants of the 

 sea are. free. Not only is there no danger of their drying up, but 

 drinking water is never a problem in the sea. Thus marine creatures 

 are freed, as it were, at a stroke, from needing any of the adapta- 

 tions requisite in this respect for life on land, and from the 

 limitations that the water supply there imposes upon regional dis- 

 persioi-s or colonizations. Free from this compelling factor, living 

 substance may, in the sea, remain naked, or may cover its outer 

 surface (in skeleton formation) with a greater variety of materials, 

 not only with minerals, as lime, silica and even compounds of 

 strontium, but vdth cellulose, agar, chitin, spongin, and various 

 solidified proteins, such as the byssus threads of molluscs, and Tvith 

 protein erudations. Here nature has been able to give free rein to 

 the process of diversification. 



Second, while air is much lighter than protoplasm or than any 

 of the derivatives of the latter, {e.£.:., bones, shells, etc.), sea 

 water is of almost the same specific gravity as protoplasm. On land, 

 because air gives so little support, the vital substances of any liv- 

 ing being of any considerable size must in sone way be protected 

 against the pull of gravity, i.e., must have a supporting framevrork; 

 otherwise,' it will collapse of its own weight. This need for suppor" 

 is a basic reason for the development of the v;ood, and of the bony 

 skeletons of land plants and animals. And while the skeleton of a 

 bird or maunrnal equally serves another purpose, and one which we r;>or- 

 usually appreciate, (the attachment of muscles), yet it is oni^'' foi^ 

 support that large animals and plants absolutely require complex 

 skeletal structures. 



T':.is necessity for support not only introduces great com- 

 plexity, but it limits size, for as size increases, so does the 

 difficulty of providing adequate support: size of land animals is 

 limir.ed by the strength of bones and sinews. 



In the sea there is no necessity for any support, no matter hoTv 

 large a marine animal or plant may be. No alga, "for example, needcs 



