Darwin was one of the popular examples of the
Meyers-Briggs INTP. As such, he was riddled with self-doubt and anxiety over his realization about the
Origin of Species. As noted by
pimephalis, he was quite a religious man, and when it became obvious that the theory he had formulated was directly contradictory to the
doctrine of the church, he was tortured by the truth he had always been taught to believe and wanted to believe, and the
truth that he now knew to be fact.
In the end, the scientist won out over the priest. Although Darwin would not live to see his theories vindicated as they are today, he clearly understood the gravity of them, and it was probably that which drove him to his supposed panic attacks, and generally made him sick with worry, which probably contributed to his rather sickly condition near the end of life.