346 EVOLUTION [CHAP. V 



Letter 255 that this very concentration of the developed characteristics 

 made room for the production of differences in the aclult 

 descendants of any given pair. Further, that in the room thus 

 made other different characteristics must be produced, and 

 that these would necessarily appear earlier in proportion as 

 the species was more or less accelerated, and be greater or less 

 in the same proportion. Finally, that in the most accelerated, 

 such as "a c" or " a d" the difference would be so great as 

 to constitute distinct genera. Cope and I have differed very 

 much, while he acknowledged the action of the accumulated 

 mode of development only when generic characteristics or 

 greater differences were produced, I saw the same mode 

 of development to be applicable in all cases and to all 

 characteristics, even to diseases. So far the facts bore us out, 

 but when we assumed that the adult differences were the 

 result of the accelerated mode of development, we were 

 perhaps upon rather insecure ground. It is evidently this 

 assumption which has led you to misunderstand the theory. 

 Cope founded his belief, that the adult characteristics were 

 also the result of acceleration, if I rightly remember it, 

 mainly upon the class of facts spoken of above in man where 

 a sudden change in two organs may produce entirely new and 

 unexpected differences in the whole organisation, and upon 

 the changes which acceleration appeared to produce in the 

 development of each succeeding species. Your difficulty in 

 understanding the theory and the observations you have made 

 show me at once what my own difficulties have been, but of 

 these I will not speak at present, as my letter is spinning 

 itself out to a fearful length. 



After speaking of Cope's comparison of acceleration and retardation 

 in evolution to the force of gravity in physical matters Mr. Hyatt 

 goes on : 



Now it [acceleration] seems to me to explain less and 

 less the origin of adult progressive characteristics or simply 

 differences, and perhaps now I shall get on faster with my 

 work. 

 Letter 256 To A. Hyatt. 



Down, Dec. I4th [1872]. 

 In reply to the above letter from Mr. Hyatt. 



Notwithstanding the kind consideration shown in your 

 last sentence, I must thank you for your interesting and 



