My dear Waterhouse

I see our argument about the Marsupiata has become almost a verbal one— I have not the slightest objection to the Monotremata & other common Marsupials being united, as I said, under one main group, & that group being called Marsupiata, if I was wrong in supposing that term already used for the common marsupials & in that case I sh^d have thought some other term ought to have been invented for the group containing the common marsupiata & monotremata. What I should, according to my small proportion of knowledge, object to, would be the putting the monotremata into the same group with the Marsupiata, not from their resemblance but from the Monotremata consisting of only two species.—¹ Even this may be right, but I revert to my old opinion, that all rules for a natural classification are futile until you can clearly explain, what you are aiming at. Until that is done, I must protest against sameness of country (as with the Monotremata) being used— And I think those, who put groups together from opinion are not so much to blame— At least Linnæus must have thought so, when he said the genus gives the characters & not the characters the genus—& yet he leaves undefined what first makes the genus which is to give the characters.— Have you read Whewell’s remarks on this subject in his History of the Inductive Sciences?—²

I have been interested by your examples from the Heteromera & Monkeys— I was aware that this was the case, but had never met such good illustrations: it is an old recognized principle with Botanists. I see no sort of objection to the existence of a natural group with not even one character in common, as long as in one division—the characters A.BC go together, in another division CDE & in third EFG, & in a fourth GHK—ie as long as they are linked indissolubly together, although in the latter links there be not one character the same as in the former.—

I believe (though why I should trouble you with my belief, which must & ought to appear the merest trash & hypothesis?) that if every organism, which ever had lived or does live, were collected together (which is impossible as only a few can have been preserved in a fossil state) a perfect series would be presented, linking all, say the Mammals, into one great, quite indivisible group—and I believe all the orders, families & genera amongst the Mammals are merely artificial terms highly useful to show the relationship of those members of the series, which have not become extinct —

But it is no use my going on this way— I beg you, however, to think clearly & define, or at least say you cannot define, (as others have done) what they are searching for by a natural classification—& when you have settled that, it will be easier to settle whether importance of the organs to life (which I suppose is meant by that vague term their value) or the country inhabited or number of species &c & should or sh^d not form elements in your system.

Do sometime tell me what are the reasons, which tend to unite Aptera & Diptera— How very curious. Do not such facts lead you to suppose, that classification is

almost independent of the characters of the groups, but is governed by the breaks or chasms in the series.—which is the view above given hypothetically by me with respect to the Mammals.— Are the Aptera & Diptera united by some nearly intermediate form.—

What do you exactly mean by low groups contrasted with high groups—when you speak of them being allowed a greater latitude of character.

I shall be curious to hear what your object is in a Natural Classification, with every term clearly defined & no metaphorical words like ⁠⟨⁠“⁠⟩⁠relationship” used, or at at least if used, explained.—

Farewell— | I shall be rejoiced to hear good news about the British Museum. | Ever yours | C. Darwin

P.S.— I return you your letter, which I had not thought of destroying.— When you have used them, please to let me have them again, & will you also keep this one letter of mine to be returned; as at some future year, I shall be curious to see, what I think now.—