398 



SYNOPSIS OF THE CYPRINTDyK OF PENNSYLVANIA. 



II. That the other extreme of such series is excessively modified and specialized, and 

 so diverging from all other forms, as to admit of no type of form beyond it* 



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 typc.f 



V. That rudimental organs are undeveloped or degraded conditions of the respective 

 characters developed or obliterated in the extreme of the series. 



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

 in those characters which characterize the extreme of that series. 



VII. That the relations of the genera of a 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 ] 1) . 



VIII. That a natural (£ e. trenchant) genus is distinguished from its nearest affines by 

 but few characters. 



IX. And that their presence, rudimental condition, or absence, can be accounted for on 

 the hypothesis of a greater rapidity of development in the individuals of the species of 

 the extreme type, such stimulus being less and less vigorous in the individuals of the 

 types as we recede from the same, or by a reversed impulse of development, where the 

 extreme is characterized by absence or "mutilation" of characters. 



X. That it therefore results that there is one primary structural type involved in such a 

 series of species, which is made to present at any given period in its Geologic history that 

 appearance of succession of genera ordained by Creative Power. 



Illustrations of this proposition are innumerable. Thus in the important character of 

 the branchiae, some salamanders lose them before birth and are born alive ; other viviparous 

 species maintain them for a short time after birth ; some arc produced from the egg, but 

 lose them very shortly after birth, without employing them functionally; others use them 

 during a very short aquatic life, some maintain them longer, and some to adult age ; 

 finally, some Amblystomata- reproduce while carrying branchiae, thus transmitting this fea- 

 ture to their offspring as an adult character. 



In both Perissodactylous and Artiodactylous Mammalia, certain types develop their 

 family character of canines at the earliest appearance of dentition, others not till a com- 

 paratively late period of life (Equus), and the extreme genera never produce them. 



Among Cetaceans the genus Orca maintains a powerful and permanent series of teeth ; 



* Leconte. Dana on Cephalization. 



t Owen on Cetacea, Trans. Zool. Soc. Lond , 18GG, 44. Leconte on Carabidse, Trans. Anier. Pliilos. Soc, 

 1853, 364. 





