IIRC, this occurs in a private letter and not in his published work. This would very strongly indicate that he did not feel the subject important to his theory.
I've read On the Origin of Species (6th edition, the one included in the Great Books of the Western World collection) and The Descent of Man (which happens to be bound with it). His style is intellectual honesty to the core--and the very opposite of that of his critics. He tells you what he thinks and why he thinks so. He retraces every bit of his own self-checking, anticipating as many objections as possible to his own ideas and then answering them. He's sharing the adventure of how he arrived at his theory. He's really laying out that he's tried to make sure of his result. "I thought of THIS, but it's not a problem for this reason. I thought of THAT, but it's not a problem because ..."
If it needed to be addressed, if it were important to anything, some discussion of abiogenesis would be in there and it isn't.
Darwin's prose is exquisite.