110 ORIGIN OF SPECIES 



other large groups of animals, pair for each birth. Modern 

 research has much diminished the number of supposed her- 

 maphrodites, and of real hermaphrodites a large number 

 pair; that is, two individuals regularly unite for reproduc- 

 tion, which is all that concerns us. But still there are many- 

 hermaphrodite animals which certainly do not habitually pair, 

 and a vast majority of plants are hermaphrodites. What 

 reason, it may be asked, is there for supposing in these cases 

 that two individuals ever concur in reproduction? As it is 

 impossible here to enter on details, I must trust to some gen- 

 eral considerations alone. 



In the first place, I have collected so large a body of facts, 

 and made so many experiments, showing, in accordance with 

 the almost universal belief of breeders, that with animals and 

 plants a cross between different varieties, or between indi- 

 viduals 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 that no organic being fertilises itself for a perpetuity 

 of generations; but that a cross with another individual is 

 occasionally — perhaps at long intervals of time — indispen- 

 sable. 



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

 understand several large classes of facts, such as the follow- 

 ing, which on any other view are inexplicable. 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 ! 

 If an occasional cross be indispensable, notwithstanding that 

 the plant's own anthers and pistil stand so near each other 

 as almost to insure self-fertilisation, the fullest freedom for 

 the entrance of pollen from another individual will explain 

 the above state of exposure of the organs. Many flowers, on 

 the other hand, have their organs of fructification closely en- 

 closed, as in the great papilionaceous or pea- family; but these 

 almost invariably present beautiful and curious adaptations 

 in relation to the visits of insects. So necessary are the visits 

 of bees to many papilionaceous flowers, that their fertility is 

 greatly diminished if these visits be prevented. Now, it is 



