Chap. IV.] OF INDIVIDUALS. 119 



the case of hermaphrodites this is far from obvious. 

 Nevertheless there is reason to believe that with all her- 

 maphrodites two individuals, either occasionally or 

 habitually, concur for the reproduction of their kind. 

 This view was long ago doubtfully suggested by Spren- 

 gel. Knight and Kolreuter. We shall presently see its 

 importance; but I must here treat the subject with ex- 

 treme brevity, though I have the materials prepared for 

 an ample discussion. All vertebrate animals, all insects, 

 and some other large groups of animals, pair for each 

 birth. Modern research has much diminished the num- 

 ber of supposed hermaphrodites, and of real hermaphro- 

 dites a large number pair; that is, two individuals regu- 

 larly 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 hermaphrodites. What reason, it may be 

 asked, is there for supposing in these cases that two in- 

 dividuals ever concur in reproduction? As it is impos- 

 sible 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 ac- 

 cordance 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 off- 

 spring; and on the other hand, that close interbreeding 

 diminishes vigour and fertility; that these facts alone in- 

 cline me to believe that it is a general law of nature that 

 no organic being fertilises itself for a perpetuity of gen- 

 erations; but that a cross with another individual is occa- 

 sionally — perhaps at long intervals of time — indispen- 

 sable. j„ 



