THE WEB OF LIFE 275 



from another plant, and sometimes that is the only possible 

 mode. It was one of Darwin's great services that he showed 

 by experiment the advantages of cross-fertilization as 

 against self-fertilization where that is possible. The plants 

 that grow from cross-fertilized seeds are more robust, tend 

 to flower earlier, and have more numerous and better seeds. 

 In Mexico the vanilla is cross-pollinated by bees ; in other 

 regions the stamen is rubbed against the pistil artificially ; 

 there is said to be no doubt as to the superiority of the 

 Mexican vanilla. Darwin also pointed out the interesting 

 fact that if there be placed on a stigma a pollen grain from 

 the same flower and a pollen grain from another plant of the 

 same species,the pollen-tube of the latter grows more rapidly 

 and usually wins the race for the ovum. If the conclusion 

 be accepted that cross-fertilization is the advantageous 

 mode, then the importance of bees and other flower- visiting 

 insects is plain, for it is they who unconsciously effect the 

 pollination. On their visits to flowers various parts of their 

 bodies are dusted with pollen from the stamens, and when 

 they pass on to other flowers of the same species they 

 mechanically and inevitably transfer the pollen to the 

 stigmas. 



If the bees are useful to the races of flowering plants 

 which they visit, as experiment proves, and if the flowers 

 are useful to the bees, as is evident, then we should on 

 general grounds expect to find a variety of adaptations 

 fitting the bees to make the most of the flowers and fitting 

 the flowers to make the most of the bees. That is what is 

 found, and it is very instructive to notice that there is, so 

 to speak, a long inclined plane of adaptiveness, some bees 

 being much fitter flower-visitors than others, and some 

 flowers making much more of the bees than do others. 



