342 EVOLUTION [CHAP. V 



Letter 254 slight degree, I think, with Prof. Cope, who does not write 

 very clearly. I think I now understand the terms " accelera- 

 tion " and " retardation " ; but will you grudge the trouble of 

 telling me, by the aid of the following illustration, whether 

 I do understand rightly ? When a fresh-water decapod 

 crustacean is born with an almost mature structure, and 

 therefore 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 a case of retardation ? If these illustrations 

 are correct, I can perceive why I have been so dull in under- 

 standing 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 occurring 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 ab abd ae, 

 -ad 



the differences between the successive species, expressed by 

 the terminal letter, was due to acceleration : now, if I under- 

 stand 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 



between the adult of one individual or set of individuals, and a transitional 

 stage of one or more other individuals. This doctrine is distinct from 

 that of an exact parallelism, which had already been stated by von Baer." 

 The last point is less definitely stated by Hyatt in his letter of 

 Dec. 4th, 1872. "I am thus perpetually led to look upon a series very 

 much as upon an individual, and think that I have found that in many 

 instances these afford parallel changes." See also Lamarck the Founder 

 of Evolution, by A. S. Packard : New York, 1901. 



