This subject has come up many times on RootsChat, and there are many posts, just put "cousins marrying" in Search.
As is well known, in 1540 parliament legalized marriages between first cousins to clear the way for the marriage of Henry VIII to Catherine Howard, although nowhere did the Bible indicate that cousin marriage was undesirable. What is perhaps not so well known is that in 1870, Darwin's close associate, John Lubbock, was elected to parliament and Darwin urged Lubbock to propose the inclusion of a question on cousin marriage in the 1871 census. It would then be established whether families in which the spouses were cousins had fewer children than the average. If so, 'we might safely infer either lessened fertility in the parents, or which is more probable, lessened vitality in the offspring.' Later it might also be possible to find out whether or not 'consanguineous marriages lead to deafness, and dumbness, blindness, etc.' The proposed census question was rejected by the House, although, in committee, the vote was forty five for and ninety two against. George Darwin, Charles's eldest son, then carried out a detailed study and found that there was no evidence that such marriages had significant deleterious consequences from the biological point of view.
[George H. Darwin, 'Note on the Marriage of First Cousins', Jl. Statistical Soc., xxxviii (1875)]
Stan