300 EVOLUTION [Chap. IV 



Letter 217 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 con- 

 ditions. 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. I have struck as fair a balance as our 

 knowledge permits. 



It is wonderful to me that you have patience to read my 

 slips, and I cannot but regret, as they are so imperfect ; they 

 must, I think, give you a wrong impression, and had I sternly 

 refused, you would perhaps have thought better of my book. 

 Every single slip is greatly altered, and I hope improved. 



With respect to the human ovule, I cannot find dimensions 

 given, though I have often seen the statement. My impression 

 is that it would be just or barely visible if placed on a clear 

 piece of glass. Huxley could answer your question at once. 



I have not been well of late, and have made slow progress, 

 but I think my book will be finished by the middle of 

 November. 



Letter 218 A. R. Wallace to C. Darwin. 



[Enrl of Feb., IS6S] 



I am in the second volume of your book, and I have 

 been astonished at the immense number of interesting facts 

 you have brought together. I read the chapter on pangenesis 

 first, for I could not wait. I can hardly tell you how much 

 I admire it. It is a positive comfort to me to have any 

 feasible explanation of a difficulty that has always been 

 haunting me, and I shall never be able to give it up till a 

 better one supplies its place, — and that I think hardly possible. 

 You have now fairly beaten Spencer on his own ground, for 

 he really offered no solution of the difficulties of the problem. 

 The incomprehensible minuteness and vast numbers of the 

 physiological germs or atoms (which themselves must be 



