The Evolution of Communities 



215 



parasites generated ecological variables which allowed both para- 

 sites to coexist. When the host density is high the parasite Neo- 

 catolaccus mamezophagtts predominates and presumably outcom- 

 petes the other parasite Heterospilus prosopidis. However, when 

 high Neocatolaccus incidence reduces the host population to a low 

 density, Heterospilus outcompetes Neocatolaccus (Fig. 91). The 



10 



20 



30 40 



GENERATION 



50 



60 



Fig. 91. Population fluctuation in a host and its two insect parasites. The 

 hollow circles and thick line indicate the host Callosobnichus chinensis. The 

 solid circles and broken line indicate the parasite Neocatolaccus mamezophagus. 

 The crosses and dotted line indicate the parasite Heterospilus prosopidis. 

 (From Utida.) 



reason for this reversal is that the Heterospilus adult parasites have 

 a greater ability to find host larvae, which live inside the beans. 

 Thus the relation between host and parasites causes an oscillation 

 in host density; one parasite excels when it is high, the other when 

 it is low. This automatic but simple system accounts for the in- 

 definite coexistence of the two competitive parasites without varia- 

 tion in "climate." 



PRIMEVAL COEXISTENCE OF DIRECT COMPETITORS 



On the basis of the experimental results and observations above, it 

 seems logical to suppose that mixtures of primeval species would 

 have coexisted, even if they were only slightly different ecologi- 

 cally. No likely environment of these creatures would have been 

 without variables, and almost certainly no two species would have 

 been exactly alike physiologically and therefore ecologically. The 

 good chance of competitive balance or "losers" winning locally, 

 plus the certain chances of dual survival when two species are 

 adapted to opposite ends of their environmental range together 

 indicate that coexistence undoubtedly occurred between primeval 

 forms of life in direct competition for space and food. 



It is to be expected, however, that in primeval communities 



