76 THE ORIGIN OF SPECIES 



the excavation of the deepest valleys or the formation of long lines 

 of inland cliffs. Natural selection acts only by the preservation 

 and accumulation of small inherited modifications, each profitable 

 to the preserved being; and as modern geology has almost banished 

 such 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 modi- 

 fication 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 (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 hermaphro- 

 dites 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 discus- 

 sion. 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 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 individuals ever concur in reproduction? As it is impos- 

 sible here to enter on details, I must trust to some general consid- 

 erations 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 vigor and fertility to the off- 

 spring; and on the other hand, that close interbreeding diminishes 

 vigor and fertility; that these facts alone incline me to believe that 

 it is a general law of nature that no organic being fertilizes itself 

 for a perpetuity of generations; but that a cross with another in- 

 dividual is occasionally — perhaps at long intervals of time — indis- 

 pensable. 



