NATURAL SELECTION 87 



when applied to the excavation of gigantic valleys 01 

 to the formation of the longest lines of inland cliffs. 

 Natural selection can act only by the preservation and 

 accumulation of infinitesimally small inherited modi- 

 fications, 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, if it be a true principle, 

 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 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 par- 

 thenogenesis) unite for each birth ; but in the case of 

 hermaphrodites this is far from obvious. Nevertheless 

 I am strongly inclined to believe that with all herma- 

 phrodites two individuals, either occasionally or habitu- 

 ally, concur for the reproduction of their kind. This 

 view was first suggested by Andrew Knight. 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 discussion. All 

 vertebrate animals, all insects, and some other large 

 groups of animals, pair for each birth. Modern re- 

 search 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 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 



