My dear Lyell

The doctrine of Pallas applied to our domestic breeds, we will say to Dogs, is as follows.¹ He believes that man tamed a wolf in one country, the jackall in another, the fox in another & so on with other species. He does not suppose that these wild species were more variable than others. He hypothetically supposes that when long domesticated they lose their tendency to sterility when crossed with the other domesticated species; & by their crossing when domesticated he believes that all our domestic races have originated. He or his followers further believe that the crossing gives as a tendency to new characters to arise, & this is not a little hypothetical. The whole doctrine is very hypothetical.² Yet I certainly believe (but cannot here give reasons) that the American domestic dogs have descended from at least 3 or 4 aboriginally distinct species, & that European dogs probably from several other species. There truly is, as you say, “an uncomfortable vague-ness” in whole doctrine.

You will see that I have spoken cautiously on subject. It is a curious, but not important subject for us: we believe that all canine species have descended from one parent; & the only question is whether the whole or only a part of difference between our domestic breeds has arisen since man domesticated them. I do not see how the question concerns specific centres.— The importance of period of gest-ation, has, I think, been much exaggerated: there is great individual difference in the period in our improved cattle, & by selection, a breed could in all probability be easily made with longer or shorter period.—

The Races of Man offer great difficulty: I do not think doctrine of Pallas, or that of Agassiz that there are several species of man, helps us in the least.—³ I fancy I have got some rays of light on subject; but mutiny in India stopped some important enquiries.⁴ Much too long a subject for letter. I will add I do not attribute much effect to climate &c.—

I believe, as you will see when you reread, that some plants migrated through Tropical lowlands during glacial period.

I am terribly lame with inflamed leg (what the Water-cure Doctors call a severe crisis)⁵ & this makes my hand-writing rather worse even than usual.—

If my book is reviewed in Quarterly, how I hope that Sir. H. Holland will not be Reviewer: he is so presumptuous & knows so little.⁶

Our difference on “principle of improvement” & “power of adaptation” is too profound for discussion by letter. If I am wrong, I am quite blind to my error. If I am right, our difference will be got over only by your rereading carefully & reflecting on my first four chapters. I supplicate you to read these again carefully.— The so-called improvement of our short-horn cattle, pigeons &c, does not presuppose or require any aboriginal “power of adaptation” or “principle of improvement”; it requires only diversified variability, & man to select or take advantage of those modifications which are useful to him: so under nature any slight modification which chances to arise & is useful to any creature is selected or preserved in the struggle for life; any modification which is injurious is destroyed or rejected; any which is neither useful nor injurious will be left a fluctuating element.—

When you contrast natural selection & “Improvement”, you seem always to overlook (for I do not see how you can deny) that every step in the natural selection of each species implies improvement in that species in relation to its conditions of life. No modification can be selected without it be an improvement or advantage. Improvement implies, I suppose, each form obtaining many parts or organs, all excellently adapted for their functions. As each species is improved, & as the number of forms will have increased if we look to the whole course of time, the organic conditions of life for other forms will become more complex; & there will be a necessity for other forms to become improved or they will be exterminated; and I can see no limit to this process of improvement without the intervention of any other & direct principle of improvement. All this seems to me quite compatable with certain forms, fitted for simple conditions, remaining unaltered, or being degraded.

If I have a second Edit. I will reiterate, “Natural Selection, & as a general consequence Natural Improvement”.—⁷

As you go as far as you do, I begin strongly to think, judging from myself, that you will go much further. How slowly the older geologists admitted your grand views on existing geological causes of change!— If at any time you think I can answer any question, it is a real pleasure to me to write.—

Yours affect^y | C. Darwin