To George Bentham   22 May [1863]¹

My dear Bentham

I am much obliged for your kind & interesting letter.² I have no fear of anything that a man like you, will say annoying me in the very least degree.³ On the other hand any approval from one, whose judgment & knowledge I have for many years so sincerely respected, will gratify me much.— The objection, which you well put, of certain forms remaining unaltered through long time & space, is no doubt formidable in appearance & to a certain extent in realility according to my judgment.—⁴ But does not the difficulty rest much on our silently assuming that we know more than we do? I have literally found nothing so difficult as to try & always remember our ignorance. I am never weary when walking in any new adjoining district or country of reflecting how absolutely ignorant we are why certain old plants are not there present, & other new ones are & others in different proportions. If we once fully feel this, then in judging the theory of natural selection, which implies that a form will remain unaltered unless some alteration be to its benefit, is it so very wonderful that some forms should change much slower & much less, & some few should have changed not at all under conditions which to us (who really know nothing what are the important conditions) seem very different.— Certainly a priori we might have anticipated that all the plants anciently introduced into Australia would have undergone some modification; but the fact that they have not been modified does not seem to me a difficulty of weight enough to shake a belief grounded on other arguments.— I have expressed myself miserably, but I am far from well today.—

I am very glad that you are going to allude to Pasteur; I was struck with infinite admiration at his work.—⁵

With cordial thanks, believe me Dear Bentham | Yours very sincerely | Ch. Darwin

In fact the belief in natural selection must at present be grounded entirely on general considerations. (1) on its being a vera causa, from the struggle for existence; & the certain geological fact that species do somehow change (2) from the analogy of change under domestication by man’s selection. (3) & chiefly from this view connecting under an intelligible point of view a host of facts.—

When we descend to details, we can prove that no one species has changed: nor can we prove that the supposed changes are beneficial which is the groundwork of the theory. Nor can we explain why some species have changed & others have not. The latter case seems to me hardly more difficult to understand precisely & in detail than the former case of supposed change. Bronn may ask in vain the old creationist school & the new school why one mouse has longer ears than another mouse—& one plant more pointed leaves than another plant.—⁶