ADVANTAGE OF CROSS-FERTILIZATION 351 



accurately, in those organs which are immediately concerned 

 with these relations, that adaptation becomes most complex and 

 precise. 



Even amongst animals, however, it would be difficult to find 

 better illustrations of accurate adaptation to highly specialized 

 conditions of the environment than those which we see in the 

 wonderful structural modifications whereby the majority of 

 flowers are adapted for pollination by insect agency. This 

 process of pollination is commonly referred to as the " fertiliza- 

 tion " of the flower, although, as we have seen in a previous 

 chapter, the real act of fertilization takes place inside the 

 so-called ovule and consists in the conjugation of the male and 

 female gametes, the sperm-cells and egg-cells. 



The great majority of flowers produce both pollen and ovules, 

 containing respectively the male and female gametes ; in other 

 words they are hermaphrodite. It is obvious that in such cases 

 we have two possibilities with regard to fertilization, for the 

 flower may either be fertilized by its own pollen or by that of 

 some other flower. It may be either " self-fertilized " or " cross- 

 fertilized," and the cross-fertilization may be effected either by 

 pollen from another flower of the same plant or by pollen from a 

 different plant of the same kind, the latter being the more 

 advantageous. 



It seems at first sight a strange thing that when a flower is 

 capable of self-fertilization such a roundabout and apparently 

 unnecessary proceeding as cross-fertilization should ever take 

 place at all. It has been shown, however, that cross-fertilization, 

 if not absolutely necessary, is at any rate of very great 

 advantage to the plant, or rather to the species, in which it 

 occurs. 



Charles Darwin experimented for eleven years on this subject, 

 and proved conclusively that cross-fertilization yields better 

 results than self-fertilization, both as regards the number of seeds 

 produced and also as regards the quality of the offspring. It 

 would be impossible to give an adequate account of his work in 

 this place, but we may briefly notice one series of experiments 

 and refer for the remainder to his classical volume on the " Cross 

 and Self Fertilization of Plants." He started with the " Con- 

 volvulus major" (Ipomcea purpurea), the flowers of which are 

 hermaphrodite and may be either cross- or self-fertilized in a 

 state of nature. It is an easy matter, by artificially conve}d^g 



