CONVERGENCE OF EVIDENCE 901 



ing perhaps from ten to twenty million generations, may have been needed 

 for the transformation of the generalized mammal of the basal Eocene 

 through the many successive faunas into the varied life of the living age, 

 Lyell regarded 20,000,000 years as a probable length of a geologic 

 period required for the transformation of species. Darwin thought 200,- 

 000,000 years too short for the accomplishment of organic evolution. It 

 would appear that they were conservative in these expressions of opinion 

 and that this expansion of geologic time should be welcomed by the 

 biologist as well as by the geologist. 



BEARING8 OF GEOLOGIC TIME ON THE PROBLEM OF STELLAR ENERGY 



The length of geologic time has a h^hly significant bearing on the 

 nature of the supply of the solar radiant energy, and, as the sun is but 

 one of the host of stars, it has a wider bearing on the whole problem of 

 the sources of stellar energy and the course of stellar evolution. 



Until the discovery of radioactivity, there seemed to be no conceivable 

 adequate source for the enormous expenditure of solar energy save in the 

 contraction of the sun^s mass, transforming the energy derived from 

 gravitational infall into radiant energy; A present shrinkage in radius 

 amounting to 200 feet per year would be sufficient to liberate enough 

 energy to balance the expenditure. On this basis Helmholtz calculated 

 in 1856 that if the sun had in past time delivered heat at its present rate 

 that it could not be more than about 20,000,000 years old. Kelvin, by 

 introducing the condition that the sun is probably very much denser in 

 its interior, considered that the age might be somewhat greater. Ritter, 

 Tait, and certain other physicists held, however, that the earth's organic 

 history could not have extended beyond 10,000,000 or 12,000,000 years. 



In view of these very restricted limits, attempts have been mlde by 

 various men to draw on the internal heat of the earth as a supplemental 

 supply. It has been argued that such a source would account in earlier 

 times for the equable climates and the absence of marked zones. A thick 

 atmosphere, moist and rich in carbon dioxide, is a further postulate, serv- 

 ing as a blanket to hold in more effectively the internal heat and maintain 

 a higher temperature of the surface. 



There are several serious obstacles to the acceptance of such a hypoth- 

 esis as an important factor in paleoclimatology. 



First, the amount of radioactivity known to exist in the crust appears 

 adequate, or more than adequate, to account for the whole emanation of 

 heat. There is no evidence, therefore, that the earth is cooling and that 

 the crust gave forth more heat in earlier times. 



