EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



715 



Gunderson and Hastings, 1944; see p. 

 251). Experiments show, in some in- 

 stances, that seeds germinate more rapidly 

 after subjection to the digestive juices of 

 birds and numerous seeds pass unharmed 

 through the alimentary canal. Some birds 

 swallow the fruit and expectorate the seeds. 

 They commonly feed young birds only with 

 tlie fruit, depositing the seeds in the vicin- 

 ity of the nest. Birds seem to be the most 

 effective of all animals in distributing seeds 

 over \vide areas, and many, such as the 

 orioles, have sharp, pointed bills adapted 

 to fruit-eating. The reciprocal evolution of 

 fruits and animals (particularly birds) 

 probably originated and expanded during 

 the Tertiary period. 



Phillips (1926) discussed the influence 

 of the feeding habits of the South African 

 wild pig {Potamochoerus choeropotamus) 

 upon the germination and dispersal of 

 forest tree seeds. Of twenty-six species of 

 tree seeds studied, about a third of the 

 species are mostly destroyed by passage 

 through the pig; about a third are assisted; 

 and about a third are partially destroyed, 

 but the survivors germinate more readily. 

 Phillips concludes that the pigs benefit the 

 forests by assisting in the dispersal and 

 germination of the seeds of a number of 

 tree species, and by providing numerous 

 improved seed beds through their "rooting" 

 activities. Harm to the forest is slight, al- 

 though it might increase if the pig popula- 

 tion increased. 



The relation between pollinating insects 

 and flowering plants, found in all terres- 

 trial communities and at the surface of 

 some fresh-water communities, affords the 

 most important example of mutualism (Mc- 

 Dougall, 1941). The adaptive interactions 

 of plants and animals with reference to pol- 

 lination are described in Chapter 17, page 

 248. Other instances of mutualism discussed 

 in this division are either less convincing 

 examples of reciprocal evolution, or, if 

 truly mutual, are less important in large 

 community systems. 



Lutz (1924, 1933) showed that flowers 

 have ultraviolet patterns, invisible to the 

 eye of man, to which polUnating insects 

 react. He discounts the theory that varia- 

 tions of flower colors are adaptations to in- 

 sects through the action of selection pres- 

 sure exerted by the insects. However, other 

 types of polUnating mechanisms in flowers 



are obviously specifically adjusted to the 

 structural and behavior characteristics of 

 particular insects, and there is every reason 

 to assume that flower color is also adaptive 

 to insects and that insects exert some selec- 

 tive pressure influencing the evolution of 

 the flowers. Some colors of plants are 

 surely nonadaptive, however— for example, 

 the turning of leaves to yellow and red in 

 the autumn— and some colors invisible to 

 man may be adaptive (Lutz, 1933a). 



Gunderson and Hastings (1944) give the 

 postulated stages of adaptation of insects 

 to flowers as follows: (1) insects generally 

 with biting mouth parts, not adapted to 

 but occasionally visiting flowers; (2) in- 

 sects partly adapted to flower visits— short- 

 tongued bees and some flies (partially ef- 

 fective in cross pollination); (3) fully 

 adapted flower visitors with both structure 

 and behavior fitted for obtaining nectar and 

 pollen, while at the same time they effect 

 cross polHnation— long-tongued bees, but- 

 terflies, and hawk-moths. Correspondingly, 

 we find a large variety of flower adapta- 

 tions to insect pollination (Ames, 1944; 

 Craigie, 1927; Rau, 1945; see also p. 249), 



Haldane (1932) discusses some of the 

 disadvantages as well as the advantages 

 accruing to the insect-pollinated plant. 

 Wind pollination is fairly eflBcient and does 

 not necessitate the energy expended in the 

 growth of petals, nectar, and odor. Insect 

 pollination must limit the distribution of 

 the plant, particularly when mutual adap- 

 tation is confined to a single species of 

 poUinator for a single species of plant. Dif- 

 ferential maturity of anther and stigma, 

 dimorphic flowers, and diecious plants in- 

 sure cross pollination, but make the trans- 

 fer of pollen more precarious. 



Selection has reversed the direction of 

 evolution in some types of flowers (Hux- 

 ley, 1942, p. 109) from animal pollination 

 toward self-polhnation and wind polUna- 

 tion. It thus may be assumed that, in such 

 cases, a delicate selective pressure exists, 

 first favoring and then suppressing mutual- 

 ism. 



In some instances mutualism may revert 

 to exploitation, as in the orchid, Cryptosty- 

 lis leptochila, that stimulates ichneumonid 

 flies (Lissopimpla semipunctata) to simu- 

 late copulation with its flowers, which re- 

 semble the female flies in form and coloi 

 (Coleman, 1927, 1938). 



