PLASTICITY AND PERMANENCY OF CHARACTERS. 323 



to be the distinguishing differences: the chief of them Avere 

 certain surface markings, the prominence and the angle formed 

 by the shell along a line called the umbonal ridge, the 

 angle formed by this umbonal ridge and the line of the cardi- 

 nal margin, and the contour shape of the shells. A careful 

 study of the characters exhibited by all the known species 

 was made, and instead of finding the new specimens to repre- 

 sent a new species, they practically represented the whole 

 genus. Every specific character which was described for the 

 known species was expressed in a series of 32 specimens. 

 One feature, of great importance in producing the shape of 

 the shell, is the angle formed by the umbonal ridge and the 

 hinge-line. This character varied regularly in the series from 

 less than 30° to over 60°, and these were also the limits of 

 difference in the described species. The geological horizon 

 in which this set of specimens occurred was probably the 

 lowest in which the genus has been seen. The specimens 

 were slightly smaller in size than most of the species de- 

 scribed from other regions, but the uniformity in size and their 

 occurrence altogether in a single block of stone, well pre- 

 served as originally imbedded, are proofs that the specimens 

 were very closely related genetically, and were not very far 

 separated from a common ancestor. The variations may be 

 assumed to have been pure variations, in the strict sense of 

 the word, that is, of common origin and possessing common 

 fertility. 



This series seems to admit of only one explanation for the 

 origin of the several species of the genus Ptychopteria — i.e., 

 the fixation, by isolation or subjection to various conditions of 

 environment, of the variable characters of the initial stage of 

 the genus as it appeared in the Panama sandstone. 



The Law of Progressive Evolution of Mammals as Formulated by 

 Osborne. — The force of the evidence of Brachiopods may be 

 weakened in the minds of some by the consideration of the 

 very low rank of these organisms in the Animal Kingdom. 

 But the same methods of minute analysis lead to like conclu- 

 sions in the study of mammals, the highest type of organic 

 structure. Professor H. F. Osborne, at the conclusion of his 

 recent address, as Vice-president of the American Association 



