THE FIRST APPEARANCE OF LIFE. 97 



been, is simply a matter of conjecture. The two 

 periods together must have been long enough to 

 account for the development of the Silurian fauna 

 out of the simplest form of animal, or the evolution 

 theory falls to the ground. Whether this appeal to 

 these enormous lost intervals is considered sufficient 

 to explain the sudden appearance of life in the 

 Silurian age, will depend largely upon one's previous 

 views as to the significance of the whole question of 

 evolution. If we wish to accept evolution, we can 

 readily believe this explanation to be satisfactory; 

 if we do not wish to do so, we can find here an 

 almost insurmountable difficulty. 



This same difficulty of time has been attacked 

 from another standpoint, by an attempt to reduce 

 to years some of the intervals. Darwin has set as 

 the lowest limit which he considers necessary for 

 the development of the now existing species, three 

 hundred millions of years. Mivart, ostensibly tak- 

 ing the facts given him by Darwin, considers that 

 twenty-five hundred millions of years is nearer the 

 limit. This estimate is, however, worthless, as the 

 very data upon which it is founded are simple 

 guesses. On the other hand, Sir Wm. Thomson has 

 concluded from cosmic phenomena, that the solidi- 

 fication of the earth took place probably not more 

 than one hundred millions of years ago. Dr. Croll 

 considers this conclusion almost indisputable. The 

 age of the inhabitable world is thus much less than 

 the necessities of Darwinism demand. 



These estimates can only be reconciled by short- 

 ening the time required for the development of 



