300 GEOLOGICAL BIOLOGY. 



reticularis and its allies, as an example of the permanency of 

 the plastic condition ; the bivalve shell Lamellibranch {Pty- 

 cJiopterid) of the Upper Devonian, illustrating the initiation 

 of the species of a genus ; and Mammals, in illustration of 

 progressiv^e evolution. 



The History of the Spirifers. — When we attempt to dis- 

 cover the laws of phylogenetic succession we are obliged to 

 consider specific and varietal characters. 



As has been already shown, the length of the geologic 

 time through which the characters of generic and higher rank 

 are exhibited is, by the Brachiopods at least, measured by 

 geologic periods : and there are series of Brachiopods extend- 

 ing through one or more geologic systems in which the ge- 

 neric characters expressed are alike, the various representatives 

 from beginning to end exhibiting differences only in the 

 lesser or specific characters. 



For the study of the history of such specific characters 

 the Spirifers may be taken as examples. The whole family 

 Spiriferidae begins, according to present knowledge, near the 

 base of the Upper Silurian, and there are two known repre- 

 sentatives in the Triassic. The genus Spirifer begins at the 

 base of the Upper Silurian, and is well represented through 

 the Silurian, Devonian, and Carboniferous. There are named 

 2 1 8 species in America. Hall in his ' ' Genera of Brachiopods " 

 recognizes over two hundred species. The species referred 

 to this genus in the Mesozoic are probably of distinct generic 

 rank ; a large number besides are defined in other countries. 

 Among the numerous species assigned to this genus there are 

 great variations in a few particulars. In the whole genus 

 there may be three hundred, or possibly four hundred, good 

 species, or forms, presenting two or more describable char- 

 acters, of which each is different from any other species. 

 When we examine the whole genus, and note the characters 

 which distinguish one species from others, and arrange the 

 characters into classified groups, as they concern separate ele- 

 ments of the shell, they may be classified as modifications of 

 a few elements of the form or structure of the shell. 



The Permanent Characters of Generic or Higher Rank. — Exam- 

 ining the successive forms of Spirifers, we observe that there 



