LETTERS OF CHARLES DARWIN. 403 



This is an egotistical note, but I have not seen a naturalist for months. 



Most sincerely and deeply do I hope that this note may find you almost 



recovered. 



To A. Hyatt. 



Down, Dec. 4th, 1872. 



I thank you sincerely for your most interesting letter. You refer 

 much too modestly to your own knowledge and judgment, as you are 

 much better fitted to throw light on your own difficult problems than 

 I am. 



It has quite annoyed me that I do not clearly understand yours 

 and Prof. Cope's views; and the fault lies in some slight degree, I 

 think, with Prof. Cope, who does not write very clearly. I think I 

 now understand the terms 'acceleration' and 'retardation'; but will 

 you grudge the trouble of telling me, by the aid of the following illus- 

 tration, whether I do understand rightly? When a fresh- water deca- 

 pod crustacean is born with an almost mature structure, and there- 

 fore does not pass, like other decapods, through the Zoea stage, is this 

 not a case of acceleration? Again, if an imaginary decapod retained, 

 when adult, many Zoea characters, would this not be case of retarda- 

 tion? If these illustrations are correct, I can perceive why I have 

 been so dull in understanding your views. I looked for something 

 else, being familiar with such cases, and classing them in my own 

 mind as simply due to the obliteration of certain larval or embryonic 

 stages. This obliteration I imagined resulted sometimes entirely from 

 that law of inheritance to which you allude; but that it in many cases 

 was aided by Natural Selection, as I inferred from such cases occur- 

 ring so frequently in terrestrial and fresh-water members of groups, 

 which retain their several embryonic stages in the sea, as long as 

 fitting conditions are present. 



Another cause of my misunderstanding was the assumption that 



in your series , 



a a D _ 



the differences between the successive species, expressed by the ter- 

 minal letter, was due to acceleration : now, if I understand rightly, this 

 is not the case; and such characters must have been independently 

 acquired by some means. 



The two newest and most interesting points in your letter (and in, 

 as far as I think, your former paper) seem to me to be about senile 

 characteristics in one species appearing in succeeding species during 

 maturity; and secondly about certain degraded characters appearing 

 in the last species of a series. You ask for my opinion: I can only 

 send the conjectured impressions which have occurred to me and which 

 are not worth writing. (It ought to be known whether the senile 



