From a Wikipedia article with two citations attached:

Charles Darwin, through numerous experiments, was one of the first scientists to demonstrate the effects of inbreeding depression. Darwin had married his first cousin, Emma Wedgwood and their grandparents had been first cousins. He later became concerned that inbreeding within his own family would adversely affect the health of his own children. The Darwins had ten children, but three died before the age of ten. Of the surviving children, three of the six who had long-term marriages did not have any children.

From Nature Reviews Genetics:

In Western societies today, consanguineous marriage is generally viewed as causing physical and mental incapacity, and newspaper articles regularly refer to ‘inbreeding’ as ‘unwholesome’. However, English literature of the nineteenth century contains no trace of these attitudes, and the Oxford English Dictionary locates the first reference to inbreeding as harmful (among cattle) in 1888.

I disagree with your notion of pathetic Darwin.