My dear Lyell

That you may not misunderstand how far I go with Pallas & his many disciples, I sh^d. like to add that, though I believe that our domestic Dogs have descended from several wild forms, & though I must think that the sterility which they would probably have evinced if crossed before being domesticated, has been eliminated, yet I go but a very little way with Pallas & Co. in their belief in the importance of the crossing & blending of the aboriginal stocks: you will see this briefly put in first chapter.—² Generally with respect to crossing, the effects may be diametrically opposite: if you cross two very distinct races, you may make (not that I believe such has often been made) a third & new intermediate race. But if you cross two exceedingly close races or two slightly different individuals of the same race, then in fact you annul & obliterate the differences.— In this latter way I believe crossing is all-important; & now for 20 years I have been working at flowers & insects under this point of view.— I do not like Hooker’s terms centripetal & centrifugal; they remind me of Forbes’ bad term of Polarity.—³

I daresay selection by man would generally work quicker than natural selection; but the important distinction between them is that man can scarcely select except external & visible characters, & secondly he selects for his own good; whereas under nature characters of all kinds are selected exclusively for each creature’s own good, & are well exerrcised: but you will find all this in Ch. IV.—

Although the Hound—Greyhound & Bull-dog may possibly have descended from 3 distinct stocks, I am convinced that their present great amount of difference is mainly due to the same causes, which have made the breeds of pigeons so different from each other, though these breeds of pigeons have all descended from one wild stock. So that the Pallasian doctrine, I look at, as of but quite secondary importance.

In my bigger book I have explained my meaning fully;⁴ whether I have in the abstract, I cannot remember.—

Yours affect— | C. Darwin