18701882] HYATT AND COPE 34! 



characteristics, and in this way it is possible to explain why Letter 253 



the young of later-occurring animals are like the adult stages 



of those which preceded them in time. If I am not mistaken 



you have intimated something of this sort also in your first 



edition, but I have not been able to find it lately. Of course 



this is a very normal condition of affairs when a series can 



be followed in this way, beginning with species a, then going 



through species a b to a b c, then a b dor a c d, and then a d e 



or simply a e, as it sometimes comes. Very often the accelera- 



tion takes place in two closely connected series, thus : 



a ab abd ae 



in which one series goes on very regularly, while another 

 lateral offshoot of a becomes d in the adult. This is an 

 actual case which can be plainly shown with the specimens in 

 hand, and has been verified in the collections here. Retardation 

 is entirely Prof. Cope's idea, but I think also easily traceable. 

 It is the opponent of acceleration, so to speak, or the opposite 

 or negative of that mode of development. Thus series may 

 occur in which, either in size or characteristics, they return to 

 former characteristics ; but a better discussion of this point 

 you will find in the little treatise which I send by the same 

 mail as this letter, " On Reversions among the Ammonites." 



To A. Hyatt. Letter 254 



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 l views ; and the fault lies in some 



1 Prof. Cope's views may be gathered from his Origin of the Fittest 

 1887 ; in this book (p. 41) is reprinted his Origin of Genera from the 

 Proc. Philadelph, A cad. Nat. Soc., 1868, which was published separately 

 by the author in 1869, and which we believe to be his first publication on 

 the subject. In the preface to the Origin of the Fittest, p. vi, he sums up 

 the chief points in the Origin of Genera under seven heads, of which the 

 following are the most important : " First, that development of new 

 characters has been accomplished by an acceleration or retardation in 

 the growth of the parts changed. . . . Second, that of exact parallelism 



