ON" THE ORIGIN OF GENERA. 91 



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. I have no doubt 

 that the field of entomology especially will furnish a great number 

 of evidences of the theory of acceleration and retardation, espe- 

 cially among the insects with active pupa3. 



Finally, having already stated the law according to which these 

 processes naturally take place, I quote the following significant 

 language of Hyatt in the above quoted essay on the Cephalopoda, 

 as approaching nearer to the ^Maw of acceleration and retarda- 

 tion " than anything I have found written. He says : 



'^'^In other words, there is an increasing concentration of the 

 adult characteristics of lower species, in the young of higher sjoe- 

 cies, and a consequent displacement of other embryonic features, 

 which had themselves, also, previously belonged to the adult peri- 

 ods of still lower forms." 



The preceding propositions have been formulated as follows, a 

 few additions being now made : 



I. That genera form series indicated by successional differences 

 of structural 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.* 



II. That one extreme of such series is a more generalized type, 

 nearly approaching in characters the corresponding extreme of 

 other series. 



III. That the other extreme of such series is excessively modi- 

 fied and specialized, and so diverging from all other forms as to 

 admit of no type of form beyond it. f 



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. J; 



V. That rudimental organs are undeveloped or degraded con- 

 ditions 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 extreme of that series. 



* St. Hilaire, Owen, Agassiz, Dumeril. 

 f Dana on " Cephalization " ; Leconte. 



X Owen on " Cetacea," " Trans. Zool. Soc," London, 1866, p. 44. Leconte on 

 " Carabidae," " Trans. Amer. Philos. Soc.," 1853, p. 364. 



