CHAP. IV.] OF INDIVIDUALS. 71 



see its importance ; but I must here treat the subject with extreme 

 brevity, though I have the materials prepared for an ample dis- 

 cussion. All vertebrate animals, all insects, and some other large 

 groups of animals, pair for each birth. Modern research has much 

 diminished the number of supposed hermaphrodites, and of real 

 hermaphrodites a large number pair ; that is, two individuals 

 regularly unite for reproduction, 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 herma- 

 phrodites. 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 

 general 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 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 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 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 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 enclosed, 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 scarcely possible 

 for insects to fly from flower to flower, and not to carry pollen 

 from one to the other, to the great good of the plant. Insects act 

 like a camel-hair pencil, and it is sufficient, to ensure fertilisation, 

 just to touch with the same brush the anthers of one flower and 

 then the stigma of another ; but it must not be supposed that bees 

 would thus produce a multitude of hybrids between distinct 



