HISTORY OF RESEARCH. cxliii 



semblances between species of different genera are of genetic origin, and therefore 

 (2) of systematic value ; (3) in any natural group the forms with relatively fewer 

 branches were developed from the more complex forms, and therefore (4) the so- 

 called "genera" are far more of a chronological than of a zoological significance. 

 She considers, however, that the various forms are "most probably the result 

 of development along certain special lines." 



According to her, therefore, there is a "Group relationship " ; for example, " all 

 the ' tuning forks ' Didymograpti have been derived from what may be termed the 

 fruticosus type of Tetragraptus, though not all from T. fruticosus itself." 



She divides them into two main groups, (1) those derived from Bryograptus, 

 (2) those derived from Clonograptus. 



In the first group there are five sub-groups : 



(a) Group containing Bryograptus ramosus var. cumbrensis, Tetrag. 



pendens, and Didymog. indentus. 

 (h) Group containing Bryograptus ramosus var. cumbrensis, Tetrag. fruti- 

 cosus, and Didymog. furcillatus. 

 (c) Group containing Bryograptus ramosus v ax. cumbrensis, Tetrag. Postle- 



thwaitii, and Didymog. bifidus. 

 (it) Group containing forms derived from Tetragraptus Bigsbyi. 

 (V) Group containing forms derived from Bryograptus Callavei. 

 In the second group there are three sub-groups : 



(a) Group containing Dichograptus octonarius, Tetrag. serra, and Didymo- 



graptus arcuatus. 

 (h) Group containing Loganograptus Logout, Didymograptus octobrachiatus, 



D. extensus, and Tetragraptus guadribrachiatus. 

 (<*/) Group containing Tetragraptus Headi and Didymograptus patulus. 

 Elles does not regard the angle of divergence of the branches as of phylogenetic 

 importance ; the mode of development has been simply in the direction of " failure 

 in dichotomous division." 



1899. 

 p . In 1898 Perner published the second section of the third 



" Etudes sur les Grap- part of his monograph on the Graptolites of Bohemia, com- 



tolites de Boheme," pleting the descriptive part of his work. It is devoted to a 



Prague, part m, description of the Graptolites of the upper part of Stage E. 



sect. b. 



Description of Species. — In the group of the Opisopodes of the genus Mono- 

 graphs, he describes and figures the well-known forms : Monog. priodon, M. 

 riccartonensis, M. latus, M. sartoriux, and M. vesiculosus, and the new species and 

 varieties M. priodon var. rimatus, var. validus, M. Jaekeli, M. unguiferus, and M. 

 Suessi. 



In the group Leptopodes Perner re-describes M. Nilssoni of Barrande and 



