GEOLOGICAL BIOLOGY. 



nian. The genus is said to be characteristic of that period ; 

 and not only in America, but in Europe, in China, in South 

 America, wherever Paleozoic rocks are known, Meristella is 

 found characteristically in the Upper Silurian, running rarely 

 a little below, but more frequently above, into the De- 

 vonian. 



There comes a time in the history of organisms of a par- 

 ticular line of descent, when a certain definite arrangement of 

 the parts of the organism becomes conspicuous, as this partic- 

 ular loop of the Meristella; the occurrence of individuals 

 developing this peculiarity is limited below and above. This 

 arrangement differs from that of the corresponding part in 

 any other animals of the same time ; and all the animals ex- 

 hibiting this character may be considered as closely allied 

 genetically, because in other characters they also show strong 

 resemblance. This state of things is evidence of the beginning 

 or initiation of a new genus. 



If all the specimens known possessing this new character 

 were examined and classified, they would be found to have 

 minor differences of form, surface marking, etc., which furnish 

 criteria for dividing them into several distinct species. Geo- 

 logically, one of these species is the first to appear; it lives 

 but a short time relatively, or it may continue to live during 

 several periods. It is peculiar to one country, or it is the 

 same throughout the world wherever the genus appears ; but 

 whether there be many or few species, the character which is 

 called a generic character begins at some particular time : 

 during a certain period it is frequently met with ; after a time 

 it ceases, and is never known to appear again. The particu- 

 lar combination of characters on some one organism consti- 

 tutes its generic characters, and we may say that the genus so 

 characterized has a certain definite life-period. 



During the Life-period of the Genus its Characters Constant. 

 While other characters may vary, these generic characters do 

 not change sufficiently to be noticed as of importance to the 

 paleontologist. Not only the generic, but the family and 

 the ordinal, characters, which are associated together under 

 the generic name Meristella, are thus constant for all the 

 specimens examined. 



