"Nothing before had ever made me thoroughly realise, though I had read various scientific books, that science consists in grouping facts so that general laws or conclusions may be drawn from them." (on his 1831 field work with Sedgwick)
"But when on shore, & wandering in the sublime forests, surrounded by views more gorgeous than even Claude ever imagined, I enjoy a delight which none but those who have experienced it can understand - If it is to be done, it must be by studying Humboldt." (to William Fox, 1832)
It is delightful to hear all that he [Gray] says on Agassiz: how very singular it is that so eminently clever a man, with such immense knowledge on many branches of Natural History, should write as he does." (to Hooker, 1854)
"Please return to me the MS., which he does not say he wishes me to publish, but I shall, of course, at once write and offer to send to any journal. So all my originality, whatever it may amount to, will be smashed." (to Lyell on Wallace, 1858)
"How singular so great a geologist should have so unphilosophical a mind!" (to Lyell on Murchison, 1859)
"He is the most astounding creature I ever encountered." (to Lyell on Owen, 1859)
"I have no great quickness of apprehension or wit which is so remarkable in some clever men, for instance Huxley."
"When I made any remark to him on geology, he never rested until he saw the whole case clearly, and often made me see it more clearly than I had done before. He would advance all possible objections to my suggestion, and even after these were exhausted would long remain dubious... On such occasions, while absorbed in thought, he would throw himself into the strangest attitudes, often resting his head on the seat of a chair, while standing up..." (on Lyell)
"...the best book of Natural History Travels ever published." (on Bates' 1863 book The Naturalist on the river Amazons)
"All that I think is that you will excite anger, and that anger so completely blinds every one that your arguments would have no chance of influencing those who are already opposed to our views." (Darwin to Haeckel, 1867)
"It was the best thing in the world for him that he went out on the Voyage of Discovery - There was some risk of his turning out an idle man: but his character will now be fixed, and if God spare his life, he will have a great name among the Naturalists of Europe." (Sedgwick, 1835)
"... a string of air bubbles." (Sedgwick on the Origin)
"This is truly monstrous!... What is the great difference between supposing that God makes variable species or that he makes laws by which species vary?" (marginal notes on Agassiz's copy of the Origin)
"Almost thou persuadest me to have been 'a hairy quadruped, of arboreal habits, furnished with a tail and pointed ears' &c." (Gray to Darwin on the Descent of Man, 1871)
For more Darwiniana, see also The Correspondence of Charles Darwin at Cambridge.