148 MARINE ANIMALS 



by way of exception, the air-breathing classes are more diverse than 

 the marine. 



The explanation of this fundamental diversification of marine life 

 lies in its much greater age. Marine representatives of all the inverte- 

 brate phyla are already present in the earliest fossil-bearing strata of 

 the Palaeozoic. The air-breathing forms appeared later and one by 

 one, though it is possible that the remains of terrestrial animals of the 

 same age have not been found or have not been preserved, on account 

 of the different conditions of fossilization on land and sea. Among the 

 vertebrates, the fishes appear first, in the late Cambrian, and the first 

 air-breathing vertebrates, the Amphibia, in the Devonian, thus still in 

 the Palaeozoic. 



This set of relationships is most simply explained by the assumption 

 that the ocean is the original home of life, and a weighty argument in 

 its favor lies in the fact that the body fluids of the marine animals 

 (with the sole exception of the highest and most recent forms, the bony 

 fishes) are isotonic with the sea water, so that no osmotic exchange 

 takes place between their body fluids and the surrounding medium, 

 which would alter the constitution of the former. Fresh-water animals, 

 on the contrary, require special modifications to prevent the dilution of 

 their body fluids by diffusion from the water in which they live, and 

 terrestrial forms require protection against the too great concentration 

 of their body fluids in consequence of the loss of water (cf. Chapter 

 IV). It appears certain that this relation in the marine forms is the 

 original one, and that in the other two we are dealing with new ac- 

 quirements or "adaptations." The waters of the ocean are the ideal 

 medium for living substance. 



One other general matter remains for brief consideration. The role 

 of bacteria in the general economy of the ocean is still a matter of con- 

 troversy. Apart from their food value, of which we know practically 

 nothing for the ocean in general, bacteria in the sea are chiefly con- 

 cerned with the decomposition of biotic residues and with the trans- 

 formation of certain simple compounds or elements. 



There are three centers of bacterial life in the sea: the marine 

 plankton, the sea bottom, and, least important, the sea water itself. In 

 fact, ordinary uncontaminated sea water in nature appears to be a 

 relatively poor medium for bacterial growth. Earlier investigators 

 plated out their samples in order to estimate numbers present ; modem 

 direct counting methods show that the bacteria are from 200 to 1000 

 times as abundant as was indicated by plating. 2 



Waksman has found that in the Gulf of Maine in water from 200 to 

 350 m., and on George's Bank in 60-75 m. of water, the numbers of 



