AQUATIC ORGANISMS AND EXTERNAL METABOLITES 507 



zooplankton. We have one clue here in the observations by 

 Marshall and Orr (1952) that successful spawning of some copepods 

 is closely dependent upon local phytoplankton growth, but it will 

 be important to know what degree of specificity this entails, among 

 other things. 



More widely ranging is Hardy's (1935) theory of animal ex- 

 clusion, which stimulated so much of the valuable work that has 

 been done in the last twenty years. Although doubtless some of 

 the phenomena which he collected together under this head may 

 now be seen as instances of general ecological or predatory balance 

 (Harvey, 1934; Beklemishev, 1957; Bainbridge, 1953), others are 

 less easily explained away. Indeed, if we view Hardy's thesis in 

 the terms of what we now know of the production cycle of many 

 algae, it would be surprising if some zooplankton organisms were 

 not adversely affected by the by-products of the later stages of an 

 algal flowering, quite apart from the diffused products of the "log" 

 phases of at least some of them (e.g., Prymnesium and Gymno- 

 dinium). Among more recent work relevant to Hardy's theory is 

 that of Ryther (1954) on the effects of algal products on the filtra- 

 tion rate of Daphnia and also that of Sieburth and Burkholder 

 (1959), which so remarkably links the antibacterial activity of some 

 Antarctic algae, via their euphausian predators, with an apparent 

 bacterial sterility of a euphausian predator, the penguin Pygoscelis 

 and other Antarctic birds. 



4. Space and time are intimately related in ecology, and some 

 of Hardy's examples were undoubtedly aspects of exclusion in 

 time, or "succession." Among others, reference must be made to 

 the interesting observations and ideas set out by Margalef (1958), 

 regarding heterogeneity and the parts played by ectocrines, 

 particularly in the later phases of succession. His view that "most 

 instances of biotic heterogeneity are cases of successional hetero- 

 chrony" is most stimulating. Similarly, the basic investigations of 

 (1) and (2) above may be expected to lead to better understanding 

 of boundary problems in pelagic ecology, yet another spatial 

 aspect of succession, and of the initiation of phytoplankton 

 outbursts as well as of phenomena of special interest such as 

 red tides. 



5. If, at first sight, the processes of succession seem somewhat 



