CONCLUSION AND DISCUSSION 397 



reasoning, may it not be worth our while to examine more closely the 

 presumptions we are accustomed to use in interpreting fossils. 



Discussion 



In reply to Mr. Ulrich's criticism, I will state that I did not intend 

 either to affirm or deny that all differences expressed by contrasted 

 faunas are the effect of changed conditions of environment, but to call 

 attention to the fact that we paleontologists are accustomed to use in our 

 interpretation of faunas the hypotheses of migration and of modification 

 without due consideration of the diametrically opposite theories regard- 

 ing the behavior of organisms when subjected to adverse conditions of 

 environment. 



When we infer from the morphologic differences presented by the spe- 

 cies of two contrasted faunas that migration has taken place, we assume 

 that a species either maintains its characters or dies when subjected to 

 adverse conditions of environment. When, on the other hand, we cite the 

 same morphologic differences as evidence of successive stages in the evo- 

 lution of the species, we are working on the hypothesis that the species 

 modifies its character either in direct response to changed conditions of 

 environment or by orthogenesis, irrespective of environment. 



It is important to know which of these conceptions of the behavior of 

 organisms is correct and applicable to the case in hand. 



To the second point, that locally slight differences in the morphology 

 of fossils are of great value in tracing the stratigraphic continuity of 

 beds, I reply certainly this is true. In attempting, however, to correlate 

 beds separated by considerable distances, and possibly in separate geologic 

 basins, such slight variations in form are of little value so long as they 

 do not exceed the limits of normal fluctuating variation expressed by 

 the species at a single horizon when represented by abundant individuals. 



In answer to Doctor White's question, I would state that the Mas- 

 carene beds across the boundary in New Brunswick are correlated with 

 the Eastport formation of the section in Washington County in the 

 southeast corner of Maine. 



To Professor Grabau's suggestion that the varietal characters expressed 

 by Strophomena rhomb oidalis arc gerontic in nature, it may be replied 

 that while it may be possible to detect in the Carboniferous some evi- 

 dences of old age characteristics, it is the fact that Prom beginning to 

 end the species expresses a great amount of variation, and the plasticity 

 of form is continued with recurrences of the species as we follow it up- 



