90 HISTORY OF THE OCEANS 



picture of salinity requirements against the background of modern 

 classification, the simple conclusion would be somewhat as follows. 



The earliest complete organisms lived in more or less interfacial 

 habitats of low but perhaps somewhat variable salinity. This type 

 of habitat remained the important one during the evolution of all 

 the Monera. Terrestrial as well as freshwater forms developed 

 during this phase, but a wholesale invasion of the oceans of such 

 a kind that could produce a specialized marine biota was delayed 

 until relatively late, and coincided with the elaboration of cyto- 

 logical organization of the sort found in the Protista and higher 

 plants and animals. 



This hypothesis is put forward merely as the simplest one 

 apparently concordent with the facts. The main objections that 

 can be raised against it are that it implies that the modern ecology 

 of organisms is comparable to their paleoecology. 



There must be good physiological reasons for the apparent 

 extreme superdispersion of invasions by marine metazoa into fresh 

 waters. It may appear, since it is reasonably certain that no 

 organism could work if the cytoplasm contained the same concen- 

 trations of major anions and cations as its medium, that some 

 regulation across the bounding membrane was essential right 

 from the beginning. Perhaps, therefore, the evolutionary eury- 

 halinity of the Monera is what one should expect. Yet enough 

 cases are known of large groups of Protista being exclusi\^ely 

 marine to suggest that there is some adaptive meaning in the 

 development of such lineages even when we are dealing with 

 single cells, so that even if euryhaline adaptability is physio- 

 logically essential in really primitive organisms, we still have to 

 explain why at a certain grade of evolution it tends to be lost. 



The time relations of all these processes raise the second of the 

 two problems that I want to consider, namely the very obvious 

 one as to why the fossilized remains of animals are almost absent 

 up to the Cambrian and then appear in great numbers. This 

 matter will be discussed by a number of other specialists. My aim 

 is merely to delimit the problem in a biologically significant way. 



It is first necessary to realize that the appearance of skeletons 

 does not imply the appearance solely of calcium carbonate skele- 



