ANIMAL AND PLANT REPBODUCTION 35 



provisions are made to use them as pollen carriers 

 through numberless modifications of calyx, corolla, sta- 

 mens and pistil ; how the animals themselves have devel- 

 oped organs for extraction of food or for attachment to 

 the blossoms (Fig. 8). Perhaps some of these mutual 

 adaptation mechanisms are a little fanciful, but the fact 

 remains that actually an occasional or a frequent cross- 

 pollination is secured by a majority of our 100,000 or more 

 species of flowering plants by means of insects, and the 

 hundreds of mechanisms by which it is obtained are wit- 

 ness of its paramount importance. 



The thesis of this chapter, then, is simple. The whole 

 trend of evolution in both animals and plants as regards 

 all the mechanisms in any way connected with reproduc- 

 tion, has been such as to provide effectively for continuous 

 descent. In the midst of strenuous competition for place, 

 those organisms which were able to cross with others, at 

 least occasionally, held such an advantage over those 

 which were compelled to continue through one single line 

 of descent, that their descendants have persisted in greater 

 numbers. They have dominated the organic world. Any 

 satisfactory interpretation of the effects of inbreeding 

 and outbreeding must permit a reasonable explanation of 

 this situation. 



