88 ON THE ORIGIN OF SPECIES 



facts, showing-, in accordance with the almost universal 

 belief of breeders, that with animals and plants a cross 

 between different varieties, or between individuals of 

 the same variety but of another strain, gives vigour 

 and fertility to the offspring ; and on the other hand, 

 that close interbreeding diminishes vigour and fertility ; 

 that these facts alone incline me to believe that it is a 

 general law of nature (utterly ignorant though we be 

 of the meaning of the law) that no organic being self- 

 fertilises itself for an eternity of generations ; but that 

 a cross with another individual is occasionally — perhaps 

 at very long intervals — indispensable. 



On the belief that this is a law of nature, we can, I 

 think, understand several large classes of facts, such 

 as the following, which on any other view are inex- 

 plicable. Every hybridizer knows how unfavourable 

 exposure to wet is to the fertilisation of a flower, yet 

 what a multitude of flowers have their anthers and 

 stigmas fully exposed to the weather ! but if an occa- 

 sional cross be indispensable, the fullest freedom for 

 the entrance of pollen from another individual will 

 explain this state of exposure, more especially as the 

 plant's own anthers and pistil generally stand so close 

 together that self-fertilisation seems almost inevitable. 

 Many flowers, on the other hand, have their organs of 

 fructification closely enclosed, as in the great papilio- 

 naceous or pea-family ; but in several, perhaps in all, 

 such flowers, there is a very curious adaptation between 

 the structure of the flower and the manner in which 

 bees suck the nectar ; for, in doing this, they either 

 push the flower's own pollen on the stigma, or bring 

 pollen from another flower. So necessary are the 

 visits of bees to papilionaceous flowers, that I have 

 found, by experiments published elsewhere, that their 

 fertility is greatly diminished if these visits be pre- 

 vented. Now, it is scarcely possible that bees should 

 fly from flower to flower, and not carry pollen from 

 one to the other, to the great good, as I believe, of the 

 plant. Bees will act like a camel-hair pencil, and it is 

 quite sufficient just to touch the anthers of one flower 



