My dear Henrietta

The subject is doubtful & obscure. In the marriage of cousins there is obviously danger from evils, possibly latent on both sides, being developed in the offspring. As far as mere consanguinity is concerned, judging from the analogy of animals, I sh^d. think no direct evil would follow from the marriage of cousins.¹ Nevertheless I sh^d. quite expect that if all the offspring of cousins throughout England could be compared with the offspring from parents not related, that the latter would be somewhat superior in size, & vigour   On the other hand, my recent observations lead me to conclude that if two cousins had long lived at some distance from each other, & had consequently been exposed to somewhat different conditions of life, this would counterbalance any slight evil from their consanguinity.—² I do not know of any reason whatever to lead to the belief that the marriage of cousins on male or female side would be more injurious, one than the other.— Double cousins, (i.e. from 2 brothers marrying 2 sisters) would of course be more nearly related than ordinary cousins, & I sh^d. expect that their offspring would tend to suffer more.— Under our present state of knowledge the injury from the increase of any bad tendency common to the family seems to me more to be feared than that from mere consanguinity; though the good effects of crossing distinct families I look at as great & undoubted.

Yours affectionately | Ch. Darwin