278 PEOCEEDINGS OF THE ACADEMY OF 



These wing characters are in many cases generic, it appears to the writer ; 

 and the fact that they differ without corresponding specific differences, is 

 important evidence as to the origin of genera. 



6. The females of the Lepidopterous genus Tliyridopteryx never develope 

 beyond the pupa state, according to the same authority, before reproduction ; 

 they are reproducing pupiv, so far as the external characters concerned in me- 

 tamorphosis go. In other words, the latter have beau retarded, while the re- 

 productive system and others have progressed. Now generic characters are 

 seen in the first, not in the last. The influence of the males is sufficient to 

 pi-event more than a part of the offspring from being retarded in the same 

 manner. 



I have selected a few of this class of facts which have come before my 

 mind during the present writing, as drawn mainly from my own experience. 

 How many more of the same. purport could be found by search through the 

 great literature of science or in the field of nature, may be readily imagined. 

 1 have no doubt that the field of Entomology especially will furnish a great 

 number of evidences of the theory of acceleration and retardation, especially 

 among the insects with active pupie. 



Finally, having already stated the law according to which these pro- 

 cesses naturally take place, I quote the following significant language of 

 Hyatt in the above quoted essay on the Cephalopoda, as approacliing nearer 

 to the "law of acceleration and retardation," than any thing 1 have found 

 written. He says : 



" In other words there is an increasing concentration of the adult charac- 

 teristics of lower species, in the young of higher species, and a consequent dis- 

 placement of other embryonic features, which had themselves, also, previously 

 belonged to the adult periods of still lower forms." 



The preceding propositions have been formulated as follows ; a few addi- 

 tions being now made : 



I. That genera form series indicated- by successional differences of struc- 

 tural character, so that one extreme of such series is very different from the 

 other, by the regular addition or subtraction of characters, step by step.f 



II. That one extreme of such series is a ijiore generalized type, nearly ap- 

 proaching in characters the corresponding extreme of other series. 



HI. That the other extreme of such series is excessively modified and spe- 

 cialized, and so diverging from all other forms as to admit of no type of form 

 beyond it.J 



IV. That the. peculiarities presented by such extremes are either only in part 

 or not at all of the nature of adaptations to the external life of the type.^ 



V. That rudimeutal organs are undeveloped or degraded conditions of the 

 respective characters developed or obliterated in the extreme of the series. 



VI. That the differences between genera of the same natural series are only 

 in the single modifications of those characters which characterize the ex- 

 treme of that series. 



VII. That the relations of the genera of a primary series, are those of the 

 different steps in the development of the individuals of the extreme genus ah 

 ovo ( Von. Baer, Agassiz) (with sometimes the addition of special adaptive 

 features? ?) 



VIII. That the presence, rudiniental condition, or absence of a given generic 

 character can be accounted for on the hypothesis of a greater rapidity of de- 



* On Insects, II, 473. 

 t .St. Hilaire, Owen, Agassiz, Dumeril. 

 X Dana on Cephalization ; Leconte. 



? fiwen on Cetaeeri, Trans. Zool. Soc, Lon., 18G0, 44. Leconte onCarabidas, Trans. .4mer. 

 Pliilos. Soc, 185:j, 304. 



[Oct. 



