76 " ON THE INTERCROSSING OF INDIVIDUALS. [Chap. IV. 



views as the excavation of a great valley by a single diluvial wave, 

 so will natural selection banish the belief of the continued creation 

 of new organic 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 anirnala 

 and plants with separated sexes, it is of course obvious that two 

 individuals must always (with the exception of the curious and 

 not well-understood cases of parthenogenesis) unite for each birth ; 

 but in the case of 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, aud 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 hermaphro- 

 dites. 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 — 

 indispensable. 



On the belief that this is a law of nature, we can, I think, under- 

 stand 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 flcwer, yet 



