142 THE ORIGIN OF SPECIES 



the earth, as illustrative of geology"; but we now seldom 

 hear the agencies which we see still at work spoken of 

 as trifling or insignificant, when used in explaining 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 sud 

 den modification in their structure. 



On the Intercrossing of Individuals 



I must here introduce a short digression. In the case 

 < ^ a nimals and plants with separated sexes, it is of course 

 obvious that two individuals must always (with the ex- 

 ception 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 Spreugel, Knight and Kdl- 

 reuter. 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 research 

 has much diminished the number of supposed hermaphro- 

 dites, and of real hermaphrodites a large number pair; 

 that is, two individuals regularly unite for reproduction, 



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