AQUATIC ORGANISMS AND EXTERNAL METABOLITES 501 



may be found between species. Whether superficially internal or 

 demonstrably external, at all evolutionary levels, sex has been 

 and is essentially an aquatic process, in which internal and or 

 external secretions play fundamental parts. 



It was never debatable that, as in these instances, whole bodies 

 secrete and excrete substances into their environment. Some of the 

 processes were so familiar, however, as to hinder speculation in 

 ecological terms, and particularly speculation on some common 

 features of such processes. These releases of metabolites range 

 from the familiar large-scale excreta, only some of whose ecological 

 functions had been acknowledged, to the most minute quantities 

 of a wide range of metabolic by-products (sex hormones and all 

 the other familiar external secretions of plants and animals, from 

 scent and sweat to mucus, etc.). There appears, however, to have 

 been little or no realization of the general implications that, after 

 release of such metabolites all become part of the environment of 

 the neighbors of that organism, as indeed do the decaying products 

 of the whole body after its death, and that organisms throughout 

 evolution have been exposed to such a changing organic milieu. 

 While some of the more obvious ecological roles of the grosser 

 metabolites were appreciated, it was not until the ecology of the 

 microorganisms had developed sufficiently that a wider historical 

 picture could be dim.ly anticipated. 



It must have been aspects of this picture that the more thought- 

 ful marine biologists of the twenties had in their minds, perhaps 

 particularly Johnstone el al. (1924) with their "group symbioses 

 on the great scale." For this is what some of us now feel entitled 

 to expect ; in isolated instances the basic evidence for their existence 

 is now available. It lies not only in the steadily increasing evidence 

 concerning the large quantities and the particular nature of the 

 innumerable organic substances to be found in aquatic media, but 

 also in the certain knowledge that they are secreted, excreted, or 

 are otherwise derived from living organisms. Further, we now 

 know that many organisms can live and grow only if supplied with 

 organic substances which others are known to produce. Vitamin 

 Bi2 or one of its analogues (e.g., Provasoli, 1958; Southcott and 

 Tarr, 1957) is at present the best known of these, but others 



