66 NEW YORK STATE MUSEUM 



these longer spines, which made itself felt at different places, but this 

 assumption is negatived by the fact that the mutations here men- 

 tioned did not persist at all, but disappeared again as suddenly as 

 they had appeared. They are therefore more probably of the nature 

 of devices to increase the equilibrium or the floating surface of the 

 rhabdosome; devices that, however, were impracticable in other 

 regards and thus led to aberrant types. 



The case here described of the independent development, in 

 separate regions, of forms that might be brought together as one 

 mutation and even as a new species, is but the expression in a smaller 

 way of a process that, acting in diverse regions upon numerous 

 species, has led to the evolution of polyphyletic genera of graptolites. 



The heterogenetic, homoeomorphous development of the most 

 important genera of the Dichograptidae from Clonograptus and 

 Bryograptus successively through Dichograptus, Tetragraptus to 

 Didymograptus has been early recognized by Nicholson and Marr 

 and their phyletic lines of species have been traced by EUes in the 

 case of the British species through these " genera " and by Ruede- 

 mann for the American species. The result of these investigations 

 is that the genera Clonograptus, Bryograptus, Dichograptus, Tetra- 

 graptus and Didymograptus are undoubtedly polyphyletic; and 

 represent stages of parallel development in many " phyla " or 

 genetic lines of species (see p. e. table of New York State Museum 

 Memoir 7, opposite p. 554) showing the phylogeny of the species of 

 American Axonolipa or earlier graptoHtes without axes, leading 

 from multiramous irregularly branching forms (Clonograptus, 

 Bryograptus) through multiramous regularly branching forms 

 (Dichograptus) to pauciramous symmetric forms (Tetragraptus 

 and Didymograptus). The explanation for this remarkable parallel- 

 ism is sought in the suggestion that symmetry in the arrangement of 

 the branches would tend to insure an equal supply of food to each 

 branch, and that the fewer the branches the greater the supply of 

 food to the entire organism. 



As in the case of the mutations of Glossograptus 

 quadrimucronatus described above, also the develop- 

 ment of the homoeomorphous groups of Dichograptus, Tetragraptus 

 and Didymograptus has been going on independently in the various 

 oceanic basins, or even different parts of the same basin ; as is sug- 

 gested by the different genetic lines observed in eastern North 

 America and in Great Britain. Nevertheless the new " genera " 

 seem to appear at the same time in the different oceans and the 

 development to be thus strictly parallel and everywhere persistently 



