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"Man scans with scrupulous care the character and pedigree of his
horses, cattle, and dogs before he matches them; but when he comes
to his own marriage he rarely, or never, takes any such care. He is
impelled by nearly the same motives as the lower animals, when they
are left to their own free choice, though he is in so far superior
to them that he highly values mental charms and virtues. On the
other hand he is strongly attracted by mere wealth or rank. Yet he
might by selection do something not only for the bodily constitution
and frame of his offspring, but for their intellectual and moral
qualities. Both sexes ought to refrain from marriage if they are in
any marked degree inferior in body or mind; but such hopes are Utopian
and will never be even partially realised until the laws of
inheritance are thoroughly known. Everyone does good service, who aids
towards this end. When the principles of breeding and inheritance
are better understood, we shall not hear ignorant members of our
legislature rejecting with scorn a plan for ascertaining whether or
not consanguineous marriages are injurious to man.
The advancement of the welfare of mankind is a most intricate
problem: all ought to refrain from marriage who cannot avoid abject
poverty for their children; for poverty is not only a great evil,
but tends to its own increase by leading to recklessness in
marriage. On the other hand, as Mr. Galton has remarked, if the
prudent avoid marriage, whilst the reckless marry, the inferior
members tend to supplant the better members of society. Man, like
every other animal, has no doubt advanced to his present high
condition through a struggle for existence consequent on his rapid
multiplication; and if he is to advance still higher, it is to be
feared that he must remain subject to a severe struggle. Otherwise
he would sink into indolence, and the more gifted men would not be
more successful in the battle of life than the less gifted. Hence
our natural rate of increase, though leading to many and obvious
evils, must not be greatly diminished by any means. There should be
open competition for all men; and the most able should not be
prevented by laws or customs from succeeding best and rearing the
largest number of offspring. Important as the struggle for existence
has been and even still is, yet as far as the highest part of man's
nature is concerned there are other agencies more important. For the
moral qualities are advanced, either directly or indirectly, much more
through the effects of habit, the reasoning powers, instruction,
religion, &c., than through natural selection; though to this latter
agency may be safely attributed the social instincts, which afforded
the basis for the development of the moral sense."

http://www.infidels.org/library/historical/charles_darwin/descent_of_man/chapter_21.html