Darwin took to questioning the theory that all creatures had been created uniquely by God from an early age. He was able to answer the most profound questions about humans without resorting to the supernatural, and in fact wondered at one point what kind of proof he would need to believe in the supernatural - that is, God and miracles. During his day there was a great deal of splitting off from the church into more unified religions, like Unitarianism and Humanism, but Darwin's lack of a belief in the afterlife, which he formed as a young man, was indeed radical. He knew the impact his book would have and took his time about putting it together, answering the questions he knew would be asked.
I find this exposition of his reasoning and experimentation especially rewarding for my own reasoning and for explanations to others, should they be needed. Darwin was, by all accounts, a warm, affectionate, attentive father, who believed strongly in treating all creatures with respect, who, when he observed instances of animal cruelty, even went to the trouble of gathering evidence, lodging complaints, and securing convictions. He was profoundly affected by the exhibition of chimpanzees by others, by the similarities between these animals and man, and by his observance of native African tribes of men.
More, he and his wife Emma (a cousin) gave their many children great freedom, quite against the common childraising standards of the day, allowing them the run of the house and grounds with great enthusiasm. Before the first child was born, they considered how they would allow their children to treat their new and expensive furniture, and decided that there was nothing wrong with wear and tear.
It seems the right book for me to be reading at this time, and it causes me to be interested in Hume (not Brit - David). I may be looking into the guy soon. Interestingly, Darwin's larger family includes other luminaries, like Ralph Vaughan Williams, the composer.