Chap. IV.] OF INDIVIDUALS. 119 



hermaphrodites this is far from obvious. Nevertheless 

 there is reason to believe that with all hermaphrodites 

 two individuals, either occasionally or habitually, concur 

 for the reproduction of their kind. This view was long 

 ago doubtfully suggested by Sprengel, Knight and 

 Kolreuter. We shall presently 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 off- 

 spring ; 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 — indis- 

 pensable. 



7 



