84 ON THE INTERCROSSING 



most perfect manner, by the continued preservation of all 

 the individuals which presented slight deviations of structure 

 mutually favorable to each other. 



I am well aware that this doctrine of natural selection, 

 exemplified in the above imaginary instances, is open to 

 the same objections which were first urged against Sir 

 Charles Lyell's noble views on " the modern changes of 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 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 excep- 

 tion of the curious and not well understood cases of partheno- 

 genesis) unite for each birth ; but in the case of hermaphro- 

 dites 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 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 sup- 

 posed 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 reproduc- 

 tion ? As it is impossible here to enter on details, I must 

 trust to some general considerations alone. 



