44 PROLOGUE I 



and extensive to justify the assertion that all 

 future philosophical and theological speculations 

 will have to accommodate themselves to some such 

 common body of established truths as the 

 following : 



1. Plants and animals have existed on our 

 planet for many hundred thousand, probably 

 millions, of years. During this time, their forms, 

 or species, have undergone a succession of changes, 

 which eventually gave rise to the species which 

 constitute the present living population of the 

 earth. There is no evidence, nor any reason to 

 suspect, that this secular process of evolution is 

 other than a part of the ordinary course of nature ; 

 there is no more ground for imagining the occur- 

 rence of supernatural intervention, at any moment 

 in the development of species in the past, than 

 there is for supposing such intervention to take 

 place, at any moment in the development of an 

 individual animal or plant, at the present day. 



2. At present, every individual animal or plant 

 commences its existence as an organism of 

 extremely simple anatomical structure ; and it 

 acquires all the complexity it ultimately possesses 

 by gradual differentiation into parts of various 

 structure and function. When a series of specific 

 forms of the same type, extending over a long 

 period of past time, is examined, the relation 

 between the earlier and the later forms is analogous 

 to that between earlier and later stages of indi- 



