134 UNIVERSITY OF MISSOURI BULLETIN 



life on the earth — of the time when conditions became favor- 

 able to its existence. According to its requirement of a fiery 

 earth that cooled down by radiation, organic life could not 

 have existed until the temperature had reached a point ap- 

 proximately the same as its existing temperature. This re- 

 quires a period of time after the first crust was formed and 

 before life could exist of very great length. Since the range 

 of temperature at which life can flourish on the earth is very 

 narrow, the change since it first began to exist has been prac- 

 tically nil. That makes necessary an early rather rapid de- 

 cline in temperature, followed by practically a cessation of 

 decline after life conditions had been reached. This makes 

 it necessary to call the existing temperature on the earth a 

 critical point in temperature decline. Such may be the case, 

 but to have it come just at the point favorable to the develop- 

 ment of life is asking a good deal of nature. 



The earliest forms of life that we encounter in geological 

 history are already highly developed forms, and, therefore, 

 the development of life preceding this time was probably 

 greater than it has been since. In other words, our geological 

 record, so far as the sedimentary record is concerned, covers 

 only half the time since the beginning of life on the earth. 

 The time, therefore, during which the earth temperature has 

 remained relatively constant is at least twice as long as that 

 of the sedimentary geological record. This requires a period 

 of rapid radiation and rapid decline of temperature, followed 

 by a sudden cessation of decline and a period, probably greater 

 than the first, during which there has been no sensible decline. 



It is unnecessary to go further. Whatever dif^culties 

 the planetesimal hypothesis may encounter in astronomy, it 

 aids greatly in explaining many of the facts of geology which 

 are seemingly unexplainable by the other hypothesis, and it 

 disposes of the cataclysmic development of a fiery globe. It 

 allows the application of the principle of uniformitarianism 



