474 J. BAERELL ORIGIN OP THE MAUCH CHUNK SHALE 



African rivers which go completely dry. It would seem that such a 

 seasonal dryness must have stimulated greatly the evolution of terres- 

 trial and air-breathing types of animals from those which could live 

 only in fresh waters. Having once gained supremacy through the greater 

 activity possible from air breathing, a period of expansive evolution would 

 naturally set in. This would be marked on the one hand by the passage 

 of the amphibians back to the regions more generously watered, which 

 they would come to dominate until the still more powerful aquatic rep- 

 tiles should. in turn dispossess them. 



On the other hand, in perhaps a closely following time, a progressive 

 evolution from these first air-breathers, themselves brought forth by the 

 alternation of wet and dry seasons, might 'be expected to lead still further 

 to groups wholly independent of a life in the water, and thus rather 

 quickly give rise to the primitive reptilian and mammalian stems. Con- 

 sidered in this light, the wide development of climates with dry seasons 

 such as seem to have characterized the Upper Devonian, and still more 

 so the Mauch Chunk epochs, may have had vital importance in stimu- 

 lating the terrestrial evolution of the previously aquatic vertebrates. In 

 support of this hypothesis may be mentioned the distinctness of the 

 reptilian and early mammalian stems, the Diapsida and Synapsida, even 

 in the Permian, indicating that the mammals did not arise from the 

 reptiles after a considerable evolution in the latter, but that they arose 

 possibly independently from amphibian ancestors at least far down in 

 the Carboniferous age. The type forms of the amphibian and higher 

 groups, being adapted to somewhat different environments, would not 

 enter into competition until the higher groups had somewhat expanded 

 and given rise to aquatic adaptations. 



Furthermore, it is to be noted that under si;ch a hypothesis no am- 

 phibian bones which have been or may be found are likely to be directly 

 ancestral to reptiles or mammals, since floodplain deposits in climates 

 with alternate wet and dry seasons are peculiarly unfavorable for the 

 preservation of animal remains. There is thus, possibly from the nature 

 of things, a permanent break in the record of the evolution of the verte- 

 brates. 



These views are avowedly hypothetical, but they suggest that in sub- 

 Carboniferous times the amphibians may have been adapted to semiarid 

 conditions as well as to those more humid, and therefore the presence of 

 their footprints in the Mauch Chunk strata argues only against high 

 aridity. 



Conclusions and comparisons. — ^We may draw, in conclusion, a general 

 view of the seasonal changes taking place over the Mauch Chunk fore- 



