BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



249 



715). Conspicuous flowers afford no dis- 

 cernible advantage to plants other than the 

 accomplishment of cross fertihzation. The 

 diversity of form and color and especially 

 the species-specificity of these characters ap- 

 pear to be derivable through natural selec- 

 tion only on the hypothesis of benefit from 

 animal associates or partners. The counter- 

 benefit offered by the plant is, in the first 

 place, a food material available in surplus. 

 The development of a large excess of pollen 

 for wind pollination, i.e., chance pollina- 

 tion, in ages antecedent to the evolution 

 of insect pollination, affords a simple ex- 

 planation of the existence of such a surplus. 

 The surplus pollen is offered entirely with- 

 out disadvantage to the plant. The insect 

 (or animal) contribution then Ues in in- 

 creased certainty of pollination, or cross 

 pollination, and particularly of transport of 

 the pollen. The capacity for movement in 

 most animals contrasts with the incapacity 

 in this respect of most plants as a major 

 difference between the two kingdoms. By 

 impressing animals into their service for 

 the transport of pollen through the round- 

 about means of random variation and nat- 

 ural selection and reciprocal evolution, this 

 very contrast between plant and animal 

 becomes fulcrum and lever for mutualistic 

 evolution. 



The modem result is that a vast number 

 of insects live (at least in their adult stage) 

 in a flower environment— some with catholic 

 breadth of taste feeding on the nectar and 

 pollen supplied by the seasonal succession 

 of flowers, others sharply limited to the 

 blooming of a single plant species. The most 

 obvious general changes on the part of 

 plants to facihtate animal transmission of 

 their pollen lie in the development of sticki- 

 ness of pollen; in the development of mono- 

 clinous flowers; in the development of struc- 

 tures that prevent self-fertilization; and in 

 structures and additional food supplies spe- 

 cifically adapted to attract animal visitors 

 to the flowers, whether monoclinous or di- 

 clinous, monoecious or dioecious. It is to be 

 noted that the mainly carbohydrate food 

 materials supplied by nectar supplement 

 the mainly nitrogenous materials of pollen. 



The separate series of structures that fit 

 insects and other animals into the role of 

 nectar and pollen feeders on one hand and 

 on the other into that of the agents of cross 

 pollination, are extremely evident and exhib- 

 it numerous instances of parallel evolution. 



Several quite distinct orders and famifies 

 of birds enter the category of flower-feeding 

 and pollen-transporting mutuafists. The 

 mainly tropical hummingbirds of the Ameri- 

 cas and the sun birds of the Old World pro- 

 vide noteworthy examples and are adapted 

 to nectar feeding by their extraordinary 

 modified whirring flight as well as by their 

 greatly elongated bills. An elaborate brush 

 on the tip of the tongue, found in the honey 

 eaters (Melliphagidae) and in the tricho- 

 glossine parrots of the Australian region, 

 serves as an efficient pollen-collecting de- 

 vice. A few species of bats and the Austra- 

 lian marsupial Tarsipes are flower visitors, 

 and the effectiveness of bats in securing 

 cross fertihzation of certain plants is reason- 

 ably attested, especially in night-blooming 

 plants. In general, birds and mammals may 

 frequently be pollen or nectar feeders with- 

 out performing any function of cross fertili- 

 zation. 



The majority of cross polHnating animals 

 are insects. These exhibit every gradation 

 from the most evidently accidental and gen- 

 erahzed relation to flower visiting, to the 

 most precisely adjusted species-specific rela 

 tions. Knuth (p. 196) summarized the 

 graded series of mutualistic reciprocal adap- 

 tation as follows: 



"1. The more specialized a flower— i.e., the 

 more complex its structural arrangements and 

 the more deeply seated its nectar— the less are 

 its insect visitors indiscriminately drawn from 

 the entire insect fauna of a district, and the 

 more do they belong to one or several similar 

 species adapted to pollination. 



"2. The flatter and more superficial the posi- 

 tion of the nectar, the more varied are the 

 visitors in different regions, and the more are 

 they indiscriminately drawn from the entire 

 insect fauna of the region in question." 



Nectar-sucking devices characterize whole 

 families and even most of the families of 

 an order— the Lepidoptera— and pollen-col- 

 lecting or pollen-bearing structures are 

 equally evident. When it is remembered 

 that in a field of alfalfa or clover, or in the 

 sweet clover masses of roadsides and rail- 

 way embankments, every floweret must be 

 visited if a good crop of seed is to be set, 

 the numbers of insects required to perform 

 this function may be appreciated. The hair- 

 brush structures that ensure the bearing of 

 pollen from flower to flower are quite dis- 

 tinct from the pollen-gathering devices 

 when the pollen is used as food; or thev 



