The Evolution of Communities 221 



mammals will eat the whole exposed plant. Similarly a host of 

 predators may utilize different parts of one of the mammals, as do 

 a great variety of intracellular, intra-organ, and external parasites, 

 and larger vertebrate predators will eat the whole animal. Indirect 

 competition has evolved in a remarkable number of peculiar in- 

 stances, discussed at length by many authors as examples of adap- 

 tive radiation. 



There is considerable weight of opinion for the belief that all 

 indirect competition is the result of direct competition, as is illus- 

 trated in the case of two mites which feed on apple (Fig, 92). 

 Data from other sources imply the same conclusion. Lack ( 1947 ) 

 studied the Darwin finches of the Galapagos Islands and marshalled 

 much evidence supporting the hypothesis that in these birds food 

 specialization was correlated with an increase in number of species 

 living together. Thus (Fig. 94) a species existing alone on one 

 island may eat a fairly general diet, whereas on another island in 

 company with a competing species, it may eat only a specialized 

 part of the wider diet while competing species feed primarily on 

 the other part. In the species of finch with the new, restricted feed- 

 ing habits certain correlated structures have apparently changed 

 also, notably the depth and strength of the beak. 



Divergence of opinion exists concerning the mechanics of this 

 specialization. Some believe that it takes place at the edges of 

 the meeting ranges of the two species and progresses as the area 

 of overlap enlarges (Brown and Wilson, 1956). These authors cite 

 as evidence the characters and the ranges of the rock nuthatches 

 Sitta neumayer and S. tephronota, illustrated by Vaurie (1950, 

 1951). In the region of overlap the beak proportions and head 

 stripes of the two species are much more different than is the case 

 in the non-overlapping parts of the range (Fig. 95). Brown and 

 Wilson infer that this morphological change is correlated with food 

 specialization brought about by a competitive relationship between 

 the two species. An almost identical phenomenon was demonstrated 

 by Blair ( 1955 ) concerning a displacement of characters associated 

 with sexual behavior in the Microhyla frog species of Texas, as 

 was explained in Chapter 7. It seems only reasonable that selection 

 pressures could exert an analogous effect on feeding habits also. 



In contrast, many potentially competing species already have 

 sympatric ranges without any sign of food specialization. If com- 

 petition later became acute, then theoretically it would lead to food 

 specializations after the competing species had formed a more or 

 less homogeneous competitive mixture. The specializations of the 

 Galapagos finches surely began in this fashion because vagrants 



