96 ON THE ADVANTAGE Chap. IV. 



beings, or of any great and sudden modification in their 

 structure. 



On the Intercrossing of Individuals. — I must here 

 introduce a short digression. In the case of animals 

 and plants with separated sexes, it is of course obvious 

 that two individuals must always unite for each birth ; 

 but in the case of hermaphrodites this is far from ob- 

 vious. Nevertheless I am strongly inclined to believe 

 that with all hermaphrodites two individuals, either 

 occasionally or habitually, concur for the reproduction of 

 their kind. This view, I may add, was first suggested 

 by Andrew Knight. We shall presently see its im- 

 portance ; but I must here treat the subject with extreme 

 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 

 number of supposed hermaphrodites, and of real her- 

 maphrodites a large number pair ; that is, two indivi- 

 duals 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 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 general considerations 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 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 



