My dear Darwin

I should have replied sooner to your last kind and interesting letters, but they reached me in the midst of my packing previous to removal here, & I have only just now got my books & papers in a get-at-able state.¹

And first, many thanks for your close observation in detecting the two absurd mistakes in the tabular headings.²

As to the former greater distinction of the North & South American faunas I think I am right.³ The Edentata being proved (as I hold) to have been mere temporary migrants into North America in the Post-Pliocene epoch, form no part of its Tertiary fauna. Yet in South America they were so enormously developed in the Pliocene epoch that we know, if there is any such thing as Evolution &c., that strange ancestral forms must have preceeded them in Miocene times.⁴

Mastodon on the other hand, represented by one or two sp. only, appears to have been a late immigrant into S. America from the North.⁵

The immense development of Ungulates (in varied families, genera, & species) in North America during the whole Tertiary Epoch is however the great feature, which assimilates it to Europe & contrasts it with S. America. True Camels, hosts of Hog-like animals, true Rhinoceroses, & hosts of ancestral Horses, all bring N. America much nearer to the Old World than it is now. Even the horse, represented in all S. America by Equus only, was probably a temporary immigrant from the N.⁶

As to extending too far the principle (yours) of the necessity of comparatively large areas for the development of varied faunas, I may have done so, but I think not.⁷ There is I think every probability that most islands &c. where a varied fauna now exists have been once more extensive,—e.g. N. Zealand Madagascar,— Where there is no such evidence (e.g. Galapagos) the fauna is very restricted.

Lastly as to want of references; I confess the justice of your criticism.⁸ But I am dreadfully unsystematic. It is my first large work involving much of the labour of others. I began with the intention of writing a comparatively short sketch,—enlarged it, and added to it, bit by bit; remodeled the tables, the headings, & almost everything else, more than once, & got my materials in such confusion that it is a wonder it has not turned out far more crooked & confused than it is. I no doubt ought to have given references; but in many cases I found the information so small & scattered, & so much had to be combined & condensed from conflicting authorities, that I hardly knew how to refer to them or where to leave off. Had I referred to all authors consulted for every fact, I should have greatly increased the bulk of the book,—while a large portion of the references would be valueless in a few years owing to later & better authorities. My experience of refering to references has generally been most unsatisfactory. One finds, nine times out of ten, the fact as stated, and nothing more; or a reference to some third work not at hand!

I wish I could get into the habit of giving chapter & verse for every fact & extract, but I am too lazy & generally in a hurry, having to consult books against time when in London for a day.

However I will try & do something to mend this matter sh^d. I have to prepare another edition.⁹

I return you Forel’s letter.¹⁰ It does not advance the question much, neither do I think it likely that even the complete observation he thinks necessary would be of much use;—because it may well be that the ova or larvæ or imagos of the beetles are not carried systematically by the ants, but only occasionally owing to some exceptional circumstances. This might produce a great effect in distribution yet be so rare as never to come under observation.

Several of your remarks in previous letters I shall carefully consider. I know that, compared with the extent of the subject, my book is in many parts crude & ill-considered;—but I thought & still think, it better to make some generalizations wherever possible, as I am not at all afraid of having to alter my views in many points of detail. I was so overwhelmed with zoological details, that I never went through the Geological Soc. Journal¹¹ as I ought to have done, & as I mean to do before writing more on the subject.

With best wishes | Believe me | Yours very faithfully | Alfred R Wallace

C. Darwin F.R.S.