CiiAr. IV. INTERCROSSING OF INDIVIDUALS. 99 



Aviiieh is all that concerns us. But still there arc many lier- 

 maphrodito animals wliicli certainly do not habitually pair, and 

 a \-ast 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 nuist trust to some general consid- 

 erations alone. 



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

 showing, in accordance with the almost universal belief of 

 breeders, that Avith animals and plants a cross between differ- 

 ent varieties, or between individuals of the same variety but 

 of another strain, gives vigor and fertility to the offspring ; 

 and on the other hand, that dose interbreeding diminishes vig- 

 or and fertility ; tliat these facts alone incline me to believe 

 that it is a general law of Nature that no organic being fertil- 

 izes itself for a perpetuity of generations ; but that a cross 

 with another individual is occasionally — perhaps at long inter- 

 vals of time — indispensable. 



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

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

 ing, which on any other view are inexplicable. Every hybrid- 

 izer knows how unfavorable exposure to wet is to the fertili- 

 zation of a flower, yet Avhat 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-fertilization, the fullest freedom for the 

 entrance of pollen from another individual Avill explain the 

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

 other hand, have their organs of fructification closely enclosed, 

 as in the great papilionaceous or pea-fainily ; but in most of 

 these flowers there is a curious adaptation between their struc- 

 ture 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 many ]iapiIionaceous flowers, that I have 

 found, by experiments published elsewhere, that their fertility 

 is greatly diminished if these visits be prevented. 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. IJees will act lik(^ a camel-hair pen- 

 cil, and it is quite sufficient just to touch the anthers of one 

 flower and then the stigma of another with the same brush to 



