ORIGIN OF SPECIES. 143 



required for the biological revolutions which we know to 

 have occurred since the Laurentian period ; and if physical 

 geology or astronomy can show that the period demanded 

 is too great, Evolution will hardly evade the difficulty by 

 shortening the time required for the conversion of one 

 species into another. At present, however, it can only be 

 said that whilst physical geology does not absolutely need 

 the time demanded by the theory of Evolution, there is 

 nothing in the facts of the former which would forbid our 

 yielding to the requirements of the latter. There are, on 

 the other hand, good grounds to be drawn from other de- 

 partments of physical science, as shown by Sir William 

 Thomson, for the belief that the period which has elapsed 

 since the introduction of life upon the earth is much below 

 that which is required by the theory of evolution by natural 

 selection. 



