THE COURSE OF BIOLOGIC EVOLUTION. 4.") 



and agreeable flavors in fruits. But these are by no means all, 

 and we must thus account for most burs, spines, thorns, and 

 other forbidding features, viscid and glandular hairs, as in the 

 sundew, and irregular and peculiar forms of leaves, especially 

 such as are seen in the pitcher-plants, and a great variety of 

 other structures not connected with the reproductive function. 

 What then are these supra-normal or illegitimate causes 

 which result in such peculiar products ? In the first place they 

 consist in special changes in the environment which are seized 

 upon to the advantage of the plant. Plants in view of their 

 stationary character, had especial need of two things, viz., 

 cross-fertilization and dissemination. Growing together without 

 power to change their position and mingle with remoter forms, 

 there was perpetual danger that close interbreeding might de- 

 teriorate or destro3 r the stock. The seeds of such stationary 

 organisms perpetually falling in the same spot tended to choke 

 one another and to weaken and restrict the species. Even- 

 normal and legitimate means of averting these two dangers 

 had been adopted by the earlier types of vegetation. The 

 spores of cryptogams and the pollen of conifers were made so 

 light that the winds would take them up and waft them to 

 great distances. Certain grasses and other herbs were endowed 

 with the peculiarity of being uprooted by the wind at the 

 proper season and blown for miles over the plains, scattering 

 their seeds. And even water had become and still remains a 

 medium for the transportation of both pollen and seed from 

 place to place and from shore to shore. But still these instru- 

 mentalities fell far short of the needs of the vegetable world in 

 these directions. At last, and nearly at the same period in the 

 earth's history, two new, and, one may almost say, unexpected 

 agencies came forward, adapted respectively to the supply of 

 these two prime necessities of the plant — viz., insects and birds. 



