352 OUTLINES OF EVOLUTIONARY BIOLOGY 



the pollen on the tip of a feather, to bring about the fertilization 

 in any way which may be desired. Ten flowers were thus self- 

 fertilized with their own pollen, while ten others were cross- 

 fertilized with pollen from a distinct plant. The crossed and 

 self-fertilized seeds thus obtained were carefully cultivated under 

 exactly the same conditions, and it was found that the plants 

 raised from cross-fertilized seeds were taller than those raised 

 from self-fertilized seeds in the proportion of 100 to 76. 



The same experiment was repeated with ten successive 

 generations of the same plants, and always with the same result 

 in favour of the cross-fertilized individuals. Moreover, it was 

 proved at the same time that the fertility of the plants produced 

 by cross-fertilization was greatly superior to that of the self- 

 fertilized plants, a much larger quantity of seed being produced. 



These results, taken in conjunction with many others of the 

 same kind, clearly proved the advantages of cross-fertilization. 

 To quote Darwin's own words : 



"It has been shown that the offspring from the 



union of two distinct individuals, especially if their progenitors 

 have been subjected to very different conditions, have an 

 immense advantage in height, weight, constitutional vigour, and 

 fertility over the self-fertilized offspring from one of the same 

 parents." 



The theoretical explanation of this advantage is not an easy 

 matter, but is probably to be sought in the admixture of two 

 distinct series of hereditary tendencies in the offspring of a 

 cross. 



In a state of nature it is a very rare thing for plants to be 

 exclusively self -fertilized, for, although self-fertilization may take 

 place in some cases to a very large extent, and although some 

 flowers are so constructed that self-fertilization alone is possible, 

 yet there is nearly always at least an occasional cross by the 

 introduction of pollen from a different plant. 



Considering the advantages which arise from cross-fertilization^, 

 we need not be surprised to find that a very large number of 

 flowers are provided with special adaptations whereby these 

 advantages may be secured to them. We may notice first certain 

 contrivances by means of which self-fertilization is more or less 

 effectually prevented and the injurious effects of perpetual close 

 inbreeding thereby avoided. These contrivances may be classed 

 under three principal heads. 



