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ANNALS NEW YORK ACADEMY OF SCIENCES 



shape of var. ruedemanni m. f. eonferta). That this variability is of 

 rather important value is illustrated by the fact that the same features 

 which characterize the varieties are to be met again, somewhat further 

 developed and more distinctly separated, in later genera ; e- g., Callo- 

 graptus, Dendrograptus, so that these features have true orthogenetic 

 significance. 27 



Considering only this point of view, we could come to the conclusion, 

 as did Moberg in 1906, that these modifications are useful for specific 

 separation. We must bear in mind, however, that all the varieties start 

 from a common center, i. e., var. acadica = forma typica of the European 

 localities; that, therefore, an unbroken chain of bridging types exists 

 which embrace all the different forms which elsewhere, when isolated, 

 were scarcely regarded as being in any way related one to another. 



Jicfpnemci 



Destnoijraptos 



Uendroarapius 



Callocjrflptas 



rectilineafum 



murrayi 



Individuals 



Fig. 3. — Range of variability of Dictyonema ffabelliforme 



Finally, any separation by time is significant only between the var. eon- 

 ferta and most of the other varieties, which all have apparently grown 

 together. For the separation of types, I regard the foregoing as of 

 varietal, but not of specific or mutational value. For this last, we may 

 regard either the saltative character or the interval of time as the essen- 

 tial point. 



As to the cause of those changes, we may find it by starting from a 

 mechanical point of view. How do sessile organisms react to a more or 

 less continuous pull? If the pull is small, there results elongation of 



27 The constantly increasing number of Dendroidea, found in the Ordovician of North 

 America, as compared with their sporadic appearance in European localities, makes it 

 probable that the continuous evolution was more likely confined to the American seas. 



