312 GEOLOGICAL BIOLOGY. 



with the same characters not modified ; the other set are 

 plastic at the first appearance, and only by degrees in the 

 course of geologic time do they become fixed and permanent. 

 It is this difference in the law of evolution of the characters, 

 as traced in historical series, that has led to the distinction of 

 the two modes of evolution, the one intrinsic, and the other 

 cxti'insic, as defined on a previous page. Intrinsic evolution is 

 conceived of as normal expansion and differentiation of the 

 organism itself from within, and is the expression, in some 

 way, of an intrinsic tendency of the particular race of organ- 

 isms. The other, extrinsic evolutioji, expresses the limitation 

 and selection exerted upon the organism from without. Varia- 

 bility is thus the morphological expression of intrinsic evolu- 

 tion, and permanency or the transmission of characters with- 

 out modification is the morphological expression of the effect 

 of extrinsic forces. 



Hall's Analysis of the Genus Spirifer and Classification of its 

 Species. — The history of the evolution of the genus Spirifer 

 may be seen from a somewhat different point of view by an 

 ■examination of the classification of American Spirifers by 

 James Hall, than whom we have no more critical observer of 

 specific differences in fossils.* Professor Hall recognizes 

 about two hundred species of Spirifers in the American 

 Palaeozoic rocks, none of which he considers worthy to be 

 regarded as even of subgeneric rank in relation to the typical 

 Spirifer stock. But there are certain groups of species natu- 

 rally associated together in successive lines which may be 

 regarded as genetically separate races, each line being char- 

 acterized by an association of common characters and differ- 

 ing from the others by the relative development or elabo- 

 ration of one or other of its characters. 



Six such principal groups are recognized, called by Hall, 

 L Radiati ; II. Lamellosi ; Fimbriati ; IV. Aperturati ; V. 

 Osteolati; VI. Glabrati. 



Range of Species of Spirifer in American Formations. — In 

 the following table the lists are arranged in such a way as to 

 show for each particular race in each group the number of 



* " An Introduction to the Study of the Genera of the Paleozoic Brachiop- 

 ■oda," — Paly. N. Y., vol. viii, part ii, fascicle i, pp. 12-40, 1893. 



