SYNOPSES OF THE CYPRINIDjE OF PENNSYLVANIA. 



399 



in Beluga the series is shed in old age, in Globiocephalus or the Caing whales they are 

 shed at middle age, while in the Balsenidse, of which the absence of teeth is an essential 

 character, these organs are developed, and absorbed during fatal life (Eschricht). 



It is well known that the Cervidge of the Old World develop a basal snag of the antler 

 at the third year; a majority of those of the New World (genera Cariacus, Subulo), 

 never develop it except in "abnormal" cases in extreme old age of the most northern 

 Cariacus (C. virginicus) ; while the South American Subulo retains to adult age, the sim- 

 ple horn of the second year of Cervus. 



Among the higher Ccrvidae, Ilusa, and Axis never assume characters beyond an equiv- 

 alent of the fourth year of Cervus. In Dama the characters are on the other hand assumed 

 more rapidly than in Cervus, its third year corresponding to the fourth of the latter, and 

 the development in after years of a broad plate of bone, with points, being substituted for 

 the addition of the corresponding snags. 



Returning to the American deer, we have Blastocerus, whose antlers are identical with 

 those of the fourth year of Cariacus.* 



In the important character of the scutellation of the tarsi among the Passerine birds, 

 the "boot" appears early in life in the highest Oscines, later in the lower, and does not 

 appear at all in the majority. In respect to the still more important feature of the long 

 posterior plates which appear very early in most Oscines, in the Myiadestes type they 

 appear late, the squamse remaining long, while the Clamatores never develop the plates, 

 not advancing beyond the infantile squamous stage. 



In reviewing these and many similar examples everywhere coming under the eye of the 

 naturalist, it is easy to perceive what would constitute a plastic and what a conserved con- 

 dition of generic, or even of specific form. Let the development of a special feature 

 be postponed to a later period of the individual's life, so that nature's intention be but 

 feebly indicated at the period of reproduction, and that feature will be but feebly repre- 

 sented in the offspring. 



That every series of forms has had its period of great multiplication, or its " protean," 

 and thus probably plastic stage, is familiar to the student of paheontology. At other 

 times than these a vis conservatrix would seem to insure a nearly entire permanency of 

 generic form. 



The above law of retardation and acceleration is, however, only proposed as a partial 

 explanation of the mode in which the progress may have occurred ; the grand law of what 

 has determined or projected the series of genera to its extreme remains untouched. That 

 it could have been a natural selection, resulting from favorable or hostile relations of sur- 

 rounding conditions, is contradicted by the facts on which Prop. IV is based. 



* Vide the Illustrations of Cuvier's 0»semens Fossil 



