45 



develop a hard woody trunk. Because protoplasm is about as heavy 

 as watsr, there is no gravitational limit to size. The only 

 theoretic limit to the size of marine animals is the necessity for 

 taking in, through the cor'iparatively small part of the surface 

 occupied by the mouth, enough food to support the entire bulk. 

 Correlated with the relief from the force of gravitation, is the 

 fact that the sea supports animals as large today as it ever has, 

 and larger than any animals that ever existed on land. The skele- 

 tons of ma,rine animals, then, serve only for external protection, 

 or for the attachment of their muscles. And comparison between the 

 framework'' of a whale, (which collapses of its own weight and 

 suffocates if stranded on the beach by the ebbing tide), and that 

 of an elephant, shows at a glance how much less is necessary in the 

 one case than "the other. In spite of their great muscular power, 

 even the largest sharks have still feebler, and wholly cartilaginous 

 skeletons without any hard bones; while an even more striking 

 example of strength without framework is afforded by the giant 

 squids, animals proverbially active, swift swiroming, and muscular, 

 though with only the rudiment of any sort of skeleton. ITo morphclo- 

 gical development of this sort would be oossible on land; in fact, 

 sharks and squids, out of the water, fall flat of their own weig;ht. 



The fact that living matter is of about the same weight as 

 water has another very important bearing, for it allows whole 

 categories of plants and animals to pass their lives swimming or 

 drifting suspended midway between surface and bottom, an ecologic 

 category that has no parallel on land. The areal distribution' of 

 life thus extends to three dimensions in the sea, whereas on land 

 the zone that is permanently habitable reaches upward only to the 

 tope of the highest trees, downward a few feet into the soil. 



The water of the sea being in constant motion, and carrying 

 a vast assemblage of living things along with it, there is also a 

 very much better chance that food will be brought by this pursly 

 mechanical process within the reach of a stationary animal in the 

 sea than is the case en land; it need merely await whatever th.-; 

 current sweeps within its reach. While, therefore, the power of 

 locomotion is so vital for land animals as a whole that we are 

 accustomed to think of animals as creatures that move, of plants 

 as those that do not, self-directed locomotion is not a basic re- 

 quirement even for carniverous anima^ls in the sea. In fact, whole 

 categories^of flesh-caters manage very .veil without it there, and 

 all gradations are to be seen there from such as swim actively to 

 those that swim or crawl for part of their life, then becoming 

 stationary (such as the barnacles), to others that are stationary 

 or practically so throughout their entire lives, such as the stalked 

 sea lilies (crinoids). And if density of aggre.<?ation, or numerical 

 strength of individuals be an index to success in the struggle for 

 existence, the oysters, mussels, clams, etc., the deep-sea crinoids, 

 tne reef corals, and the sponges find a stationary life highly 

 successful. This ap-olies, furthermore, not only to sm.all animals, 

 but to come of considerable size, such as the giant clam (Tridacna) 

 of the coral reefs. 



The constant circulation of the sea water also affords a pui^ely 

 mechanical means of transportation, whereby the problems of dis- 

 persal are solved for the floating animals and plants that are 



