My dear Lyell

With respect to the points in y^r note I may sometimes have expressed myself with ambiguity.² At the end of Chap. 23 where I say “that marked races are not (often (you omit ”often“)) produced by changed conditions”, I intended to refer to the direct action of such conditions in causing variation, & not as leading to the preservation or destruction of certain forms.³ There is as wide a difference in these two respects, as between voluntary selection by man & the causes which induce variability. I have somewhere in my book referred to the close connection between Nat. Selection & the action of external conditions in the sense which you specify in your note. And in this sense all Natural selection may be said to depend on changed conditions.⁴

In the “Origin” I think I have underrated (& from the cause which you mention) the effects of the direct action of external conditions in producing varieties; but I hope in Chap. 23 I have struck as fair a balance as our knowledge permits.⁵

It is wonderful to me that you have patience to read my slips & I cannot but regret it, as they are so imperfect, they must I think give you a wrong impression; & had I sternly refused, you would perhaps have thought better of my book. Every single slip is greatly altered & I hope improved.—

With respect to the human ovule, I cannot find dimensions given, though I have often seen the statement.⁶ My impression is that it w^d. be just or barely visible if placed on clear piece of glass.— Huxley⁷ could answer your question at once.— I have not been well of late, & have made slow progress, but I think my book will be finished by middle of November.

Yours affectionately | C. Darwin