EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 471 



fear, as Colman says, " some stray marsupial might get his feet wet 

 in migrating to a new habitat," that I copy here his thesis and con- 

 clusions. 



" Thesis. 



" I. Secular climatic change has been an important factor in the evolu- 

 tion of land vertebrates and the principal known cause of their present dis- 

 tribution. 



"2. The principal lines of migration in later geological epochs have been 

 radial from Holarctic centers of dispersal. 



" 3. The geographic changes required to explain the present distribution 

 of land vertebrates are not extensive and for the most part do not affect the 

 permanence of the oceans as defined by the continental shelf. 



"4. The theories of alternations of moist and uniform with arid and 

 zonal climates, as elaborated by Chamberlin, are in exact accord with the 

 course of evolution of land vertebrates, when interpreted with due allow- 

 ance for the probable gaps in the record. 



" 5. The numerous hypothetical land bridges in temperate, tropical and 

 southern regions, connecting continents now separated by deep oceans, which 

 have been advocated by various authors, are improbable and unnecessary 

 to explain geographic distribution. On the contrary, the known facts point 

 distinctly to a general permanency of continental outlines during the later 

 epochs of geologic time, provided that due allowance be made for the known 

 or probable gaps in our knowledge. 



" Summary of Evidence. 



" The geologic evidence for the general permanency of the abyssal oceans 

 is overwhelmingly strong. The continental and oceanic areas are now main- 

 tained at their different levels chiefly through isostatic balance, and it is 

 difficult to believe that they could formerly have been reversed to any ex- 

 tensive degree. The floor of the ocean differs notably in its relief from the 

 surfaces of the continents, and only in a few limited areas is the relief 

 suggestive of former elevation above sea-level. The continental shelf is so 

 marked, obvious and universal a feature of the earth's surface that it affords 

 the strongest kind of evidence of the antiquity of the ocean basins and the 

 limits beyond which the continents have not extended. The supposed evi- 

 dence for greater elevation in the erosion channels across its margin have 

 been shown to be better interpreted as due to ' continental creep.' The 

 marine formations now found in continental areas have all been deposited 

 in shallow seas. No abyssal deposits have ever been certainly recognized 

 among the geologic formations of the continental platform." 



It would thus seem possible that with the continuous escape of 

 heat and volatile bodies a slight constant tendency of the earth 

 toward tetrahedral deformation might combine with the other more 



