xvi PREFACE 



must be regarded as the utmost limit of time that 

 can be assigned to life upon the earth's surface since 

 it became sufficiently refrigerated to admit of any 

 kind of life existing, would be far too short for the 

 action of Natural Selection to achieve the enormous 

 amount of differentiation that exists between the 

 multitudinous forms of life upon the globe. 



When we set ourselves to consider the slowness of 

 the processes of Natural Selection, we cannot but think 

 that those who hold this opinion are more than 

 justified. 



In a letter written on the 29th January 1906 to 

 Professor Orr of Glasgow, Lord Kelvin says : " You 

 will find a good deal more on the subject of later date 

 than the passage to which you refer, in pages 1 to 131, 

 volume ii., of my Popular Lectures and Addresses, 

 and in Phil Mag., 1899, first half-year, page 66, making 

 a strong body of evidence that the age of the earth 

 as an abode fitted for life cannot probably be vastly 

 greater than twenty million years." 



If we accept Lord Kelvin's estimate of the prob- 

 able lapse of time from the first beginning of life till 

 the present day, and also give credence to Darwin's 

 account of the processes of Natural Selection, and take 

 into consideration the vastness of the space necessary 

 to be traversed in the evolution of the fish into a 

 mammal by a series of infinitesimal accretions, each 

 of secular duration, the period of twenty million years 

 seems scarcely too long for the achievement of this one 

 evolution. 



