X PREFACE 



from a very remote epoch of geological time, the 

 earth has been peopled by a continual succession 

 of the higher fomis of animals and plants, these 

 either must have been created, or they have arisen 

 by evolution. And in respect of certain groups of 

 animals, the well-established facts of paleontology 

 leave no rational doubt that they arose by the 

 latter method. 



In the second place, there are no data what- 

 ever, which justify the biologist in assigning 

 any, even approximately definite, period of time, 

 either long or short, to the evolution of one 

 species from another by the process of variation 

 and selection. In the ninth of the following 

 essays, I have taken pains to prove that the change 

 of animals has gone on at very different rates in 

 different groups of living beings ; that some types 

 have persisted with little change from the paleo- 

 zoic epoch till now, while others have changed 

 rapidly within the limits of an epoch. In 1862 

 (see below p. 303, 304) in 1863 (vol. II., p. 461) 

 and again in 1864 (ibid., p. 89 91) I argued, not 

 as a matter of speculation, but, from paleonto- 

 logical facts, the bearing of which I believe, up to 

 that time, had not been shown, that any ade- 

 quate hypothesis of the causes of evolution must 

 be consistent with progression, stationariness and 

 retrogression, of the same type at different epochs ; 

 of different types in the same epoch; and that 

 Darwin's hypothesis fulfilled these conditions. 



