214 THE GENESIS OF SPECIES. [CHAP. 



incestuous unions to disappear by degrees, in favour of the 

 descendants (greater both in number and strength) of indi- 

 viduals who naturally, from some cause or other, as he 

 suggests, preferred to mate with strangers rather than with 

 close blood-relations; this preference being transmitted 

 and becoming thus instinctive, or habitual, in remote 

 descendants. 



But on Mr. Darwin's own ground, it may be objected 

 that this notion fails to account for "abhorrence." and 

 " moral reprobation ; " for, as no stream can rise higher 

 than its source, the original " slight feeling " which was 

 -useful would have been perpetuated, but would never have 

 been augmented beyond the degree requisite to ensure 

 this beneficial preference, and therefore would not certainly 

 have become magnified into "abhorrence." It will not 

 do to assume that the union of males and females, each 

 possessing the required " slight feeling," must give rise to 

 offspring with an intensified feeling of the same kind ; for, 

 apart from reversion, Mr. Darwin has called attention to 

 the unexpected modifications which sometimes result from 

 the union of similarly constituted parents. Thus, for 

 example, he tells us : 1 " If two top-knotted canaries are 

 matched, the young, instead of having very fine top-knots, 

 are generally bald." From examples of this kind, it is 

 fair, on Darwinian principles, to infer that the union of 

 parents who possessed a similar inherited aversion might 

 result in phenomena quite other than the augmentation of 

 such aversion, even if the two aversions should be alto- 

 gether of the same kind ; while, very probably, they might 

 be so different in their nature as to tend to neutralize each 

 other. Besides, the union of parents so similarly emotional 



1 "Animals and Plants under Domestication," vol. i. p. 295. 



