Darwin seemed always afraid, such was his sensitive and generous nature, that he did not sufficiently acknowledge his indebtedness to Lyell. He wrote to his friend in 1845:
'I have long wished not so much for your sake as for my own feelings of honesty, to acknowledge more plainly than by mere reference, how much I geologically owe you. Those authors, however, who like you educate people's minds as well as teach them special facts, can never, I should think, have full justice done them except by posterity, for the mind thus insensibly improved can hardly perceive its own upward ascent.'
Very heartily, as I can bear witness from long intercourse with him, was this deep affection of Darwin reciprocated by the man who was addressed by him in his letters as 'Your affectionate pupil.' But a stranger who conversed with Lyell would have thought that he was the junior and a disciple; so profound was his reverence for the genius of Darwin.
There can be no doubt that Lyell's extreme caution in statement, and his candour in admitting and replying to objections, had much to do with his acquirement of that authority with general, no less than with scientific, readers, which he so long enjoyed. In his candour he resembled his friend Darwin; but his caution was carried so far that, even after full conviction had entered his mind on a subject, he would still hesitate to avow that conviction. He was always obsessed by a feeling that there still might be objections, which he had not foreseen and met, and therefore felt it unsafe to declare himself. No doubt the peculiarly trying circumstances under which his work was written—a seemingly hopeless protest against ideas held unswervingly by teachers and fellow-workers—led to the creation in him of this habit of mind.
Darwin, with all his candour, was of a far more sanguine and optimistic temperament than Lyell, and the difference between them, in this respect, often comes out in their correspondence.
Thus Darwin, from the horrors he had witnessed in South America, had come to entertain a most fanatical hatred of slavery—his abhorrence of which he used to express in most unmeasured terms. Lyell, in his travels in the Southern United States, was equally convinced of the undesirability of the institution; but he thought it just to state the grounds on which it was defended, by those who had been his hosts in the Slave-states. Even this, however, was too much for Darwin, and he felt that he must 'explode' to his friend 'How could you relate so placidly that atrocious sentiment' (it was of course only quoted by Lyell) 'about separating children from their parents; and in the next page speak of being distressed at the whites not having prospered: I assure you the contrast made me exclaim out. But I have broken my intention (that is not to write about the matter), so no more of this odious deadly subject[69].'
It was just the same in their mode of viewing scientific questions. Thus in 1838, while they were in the midst of the fierce battle with the 'Old Guard' at the Geological Society, Lyell wrote to his brother-in-arms as follows:—
'I really find, when bringing up my Preliminary Essays in Principles to the science of the present day, so far as I know it, that the great outline, and even most of the details, stand so uninjured, and in many cases they are so much strengthened by new discoveries, especially by yours, that we may begin to hope that the great principles there insisted on will stand the test of new discoveries[70].'
To which the younger and more ardent Darwin warmly replied:—