At the end of Chapter XXIII., where I say that marked races are not often (you omit "often") produced by changed conditions (217/1.

"Hence, although it must be admitted that new conditions of life do sometimes definitely affect organic beings, it may be doubted whether well-marked races have often been produced by the direct action of changed conditions without the aid of selection either by man or nature." ("Animals and Plants," Volume II., page 292, 1868.)), I intended to refer to the direct action of such conditions in causing variation, and not as leading to the preservation or destruction of certain forms.

There is as wide a difference in these two respects as between voluntary selection by man and the causes which induce variability.

I have somewhere in my book referred to the close connection between Natural Selection and 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.
In the "Origin" I think I have underrated (and from the cause which you mention) the effects of the direct action of external conditions in producing varieties; but I hope in Chapter XXIII.