178 Plant Life and Evolution 



ers in attracting these, and it is quite possible that 

 the assumption of the keen power of discrimination 

 of different colors and markings which has been 

 attributed to insects has been exaggerated; but the 

 evidence is overwhelming that there is a direct con- 

 nection between the development of showy flowers, 

 and cross-pollination through insect agency. 



Cross-pollination. While many showy flowers, 

 when insect visits are prevented, pollinate them- 

 selves, there are very many in which cross-pollina- 

 tion is absolutely indispensable owing to mechanical 

 contrivances by which self-pollination is rendered 

 impossible. Some of these will be discussed more 

 at length in a future chapter. Where specializa- 

 tion reaches its extreme, pollination may depend 

 upon a single species of insect, as for instance, in 

 certain orchids and species of Yucca. 



Insects as Agents in Pollination. One group of 

 animals has played a very important role in the evo- 

 lution of the flower of the angiosperms. These 

 are the insects, the largest group of animals, bearing 

 somewhat the same relation, in point of numbers, 

 to the animal kingdom that the angiosperms do to 

 plants. Vast numbers of insects are dependent upon 

 plants for their existence, and many of the peculiar 

 modifications of their structures are unquestionably 

 correlated directly with the structures of angio- 

 spermous plants; and the modifications of the flow- 

 ering plants and insects have presumably gone on 

 side by side, each affecting the other. Thus the pe- 



