434 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



Passing back to the hypothetical consequences of another method of 

 evolving air-breathing, if there had been a highly efficient respiratory- 

 circulatory system in existence from the beginnings of air-breathing in 

 the Silurian and Devonian, the development of mentality would then 

 have received a great stimulus. The whole subsequent career of land 

 vertebrates would have been altered. The actual results are from such a 

 complex interaction of external and internal changes that the exact con- 

 sequences of such a postulate can not be predicted. It would seem, well 

 within the limits of possibility, however, for that first era of stupidity, 

 bulk, and blind ferocity to have already reached its culmination and met 

 its fate in that world revolution which closed the Paleozoic. It is possible 

 that an era of warm-bloodedness and its train of consequences would 

 have begun in the early Mesozoic instead of waiting for long geologic 

 ages, waiting on the slow perfecting of the reorganized circulatory system. 

 If, in addition, a more rapidly evolving brain had been able in air-breath- 

 ing vertebrates to utilize more generally a forward pair of limbs to serve 

 the mind, it is seen how widely different the history of land vertebrates 

 might have been. 



CONCLUSION ON THE DANGERS WHICH HAVE THREATENED ORGANIC 



PROGRESS 



But there is another side, not so pessimistic, supplemental to this pic- 

 ture of what might have been. So far as evolutionary history has been 

 interpreted, there is found no indication that low and isolated lands with 

 monotonous history, such as illustrated best by Australia in later geologic 

 times, would ever have carried evolution forward to its fruition in intel- 

 lectual life. Yet in the earlier Paleozoic ages the northern lands, flooded 

 by shallow seas, were often of this character. Contrast with this geo- 

 logical monotony, which once seemed likely to endure without end, the 

 expansion and diversification of the land surfaces through later geologic 

 times, that oscillation of conditions which is seen to have stimulated 

 progress. 



Or, again, one vast and monotonous land surface, such as is seen on the 

 planet Mars, is not regarded as an environment well adapted to stimulate 

 marked advances in evolution. The waves of progress have required first 

 restriction and isolation with wide variations of conditions, so that unlike 

 faunas could be produced and the better types acquire dominance Avithin 

 a limited habitat. Second is required the migration, mingling, and 

 competition of faunas. Judged by these principles, the post-Silurian 

 history of the northern hemisphere is seen to have been increasingly 

 favorable for the evolution of the higher types of land life. But that 



