BIOLOGICAL SCIENCE 55 



afterwards disclosed the existence of multitudes of 

 micro-organisms, hitherto unsuspected. 



But with all these studies of existing plants and 

 animals it was long before men fully realised that 

 authentic traces of extinct forms of life were to be 

 found in the fossils embedded in the rocks. Isolated 

 glimpses of this truth are seen here and there 

 in the fifteenth and following centuries, but it was 

 not till the eighteenth century that this view was 

 consistently advocated. Hutton's (1726-1797) Theory 

 of the Earth, published in 1785, taught that pro- 

 cesses still going on were adequate to explain the 

 formation of the stratified rocks, and the existence 

 of embedded fossils. But even then no general agree- 

 ment followed, and it was not till Lyell (1797-1875) 

 collected all the evidence that had accumulated in 

 his Principles of Geology (1830-33) that men of science 

 were convinced, and realised that ages must have 

 elapsed in geological processes, beside which the few 

 thousand years of the received Biblical chronology 

 were almost as nothing. 



The long series of fossil animals and plants of 

 gradually increasing complexity, found by geologists 

 in strata of different ages, raised once more the ques- 

 tion of the evolution of species. The idea of the de- 

 velopment of all existing forms of life from a few 

 simple types had been held by some of the Greek 

 philosophers, but it had vanished in the ascendancy 

 of the Biblical story of the Creation, and been dis- 



