In fact, according to this school, the idea of "right" is only the result of the gradual accretion of useful predilections which, from time to time, arose in a series of ancestors naturally selected.

In this way, "morality" is, as it were, the congealed past experience of the race, and "virtue" becomes no more than a sort of "retrieving," which the thus improved human animal practises by a perfected and inherited habit, regardless of self-gratification, just as the brute animal has acquired the habit of seeking prey and bringing it to his master, instead of devouring it {190} himself.
Though Mr.Darwin has not as yet expressly advocated this view, yet some remarks made by him appear to show his disposition to sympathise with it.
Thus, in his work on "Animals and Plants under Domestication,"[203] he asserts that "the savages of Australia and South America hold the crime of incest in abhorrence;" but he considers that this abhorrence has probably arisen by "Natural Selection," the ill effects of close interbreeding causing the less numerous and less healthy offspring of incestuous unions to disappear by degrees, in favour of the descendants (greater both in number and strength) of individuals 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 maybe 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:[204] "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 {191} 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 altogether similar; 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 would be rare indeed amongst savages, where marriages would be owing to almost anything rather than to congeniality of mind between the spouses.

Mr.Wallace tells us,[205] that they choose their wives for "rude health and physical beauty," and this is just what might be naturally supposed.

Again, we must bear in mind the necessity there is that _many individuals_ should be similarly and simultaneously affected with this aversion from consanguineous unions; as we have seen in the second chapter, how infallibly variations presented by only a few individuals, tend to be eliminated by mere force of numbers.