140 C. E. Lucas 



speculations of Akehurst (1931) about the temporal alternation of oil-producing and 

 starch-producing algae in ponds, and some tentative experiments (Lucas, 1936) 

 regarding " animal exclusion ". With these were coupled Allen and Nelson's 

 pioneer demonstrations (1910) of the need for some accessory substances in diatom 

 culture; the use of " soil solution " for more effective growth; and Gran's observa- 

 tions (1931) on the relatively intensive growth of phytoplankton at the junction of 

 two bodies of water. But more evidence was needed. 



More evidence did in fact exist, scattered widely through the field of biological 

 research, and work during the war brought even more, particularly that associated 

 with antibiotics. It thus became possible to review a wider field and to formulate the 

 concept more precisely (Lucas, 1947, and from a more general ecological viewpoint, 

 1949). In the aquatic field reference was made, among others, to the very different 

 growth-rates of diatoms cultured in different natural sea waters (Matsudiara, 1939; 

 Harvey, 1939), and Harvey's associated investigations of the effects of various 

 accessory substances on growth; to other examples of the influence of waters pre- 

 viously containing living organisms (or their by-products) upon different organisms 

 in culture (e.g. Levring, 1945); and to Fox's demonstrations of the occurrence of 

 carotenoids free in natural waters and their deposits (e.g. 1944). When supported by 

 the rapidly increasing knowledge from the various fields of microbiology, together 

 with a wide range of other biological references, it seemed not unreasonable to come 

 to conclusions along the following lines: 



" (1) It is characteristic of cells to liberate certain metabolites, and these are known 

 in a variety of instances to have great influence as endocrines. 



" (2) It is now well known that a number of these potent metabolites are eliminated 

 as secretions or excretions by the organisms themselves, and many other chemicals 

 are eliminated which are not yet known to have any specific effects within the body. 



"(3) Particularly insofar as any of these metabolites are . . . parts of the environ- 

 ment of other organisms, they may be expected to have immediate potency for many 

 of them. . . . The term ' ectocrines ' has been suggested for such metabolites. 



"(4) More generally, however, . . . the capacity for adaptation of most organisms 

 suggests that further differentiation between beneficial and antagonistic relationships 

 would be likely to have developed between the producers and those affected. In the 

 extreme instances escape, exclusion, or death must be expected on the one hand, and 

 obligatory association (parasitism, symbiosis) on the other. 



" (5) Such processes are believed to be important in evolution, and they are con- 

 sidered to mediate communal relationships in ecology, which is the contemporary 

 aspect of evolution. They should be seen as part of the nexus which also includes 

 physical and chemical relationships as well as those of prey and predator " (Lucas, 

 1949, pp. 353-354). 



If these seemed then to be rather premature speculations, they now have much 

 more specific support. It is not possible to review the whole field here and this note 

 is simply intended to bring together a few references indicating some of the lines along 

 which progress is now being made. However, the suggestions could properly be 

 regarded as stemming from, and partly supported by, the theory of " animal 

 exclusion ", and evidence (Bainbridge, 1952) has recently been brought to bear against 

 the only laboratory experiments (Lucas, 1936) specifically made to investigate that 

 theory. It seems necessary, therefore, to mention Bainbridge's experiments first, 



