CHAP. I.] INTRODUCTION. 11 



We must remember, however, that the rocks of 

 the Silurian system, overlaid by ten miles' thickness 

 of sediment entombing a hundred successive faunae, 

 each as rich and varied as the fauna of the present 

 day, themselves teem with fossils fully representing 

 all the existing classes of animals, except perhaps 

 the highest. 



If it be possible to imagine that this marvellous 

 manifestation of Eternal Power and Wisdom involved 

 in living nature can have been worked out through 

 the law of ' descent with modification ' alone, we 

 shall certainly require from the Physicists the longest 

 row of cyphers which they can afford. 



Now, although the admission of a doctrine of evolu- 

 tion must affect greatly our conception of the origin 

 and rationale of so-called specific centres, it does not 

 practically affect the question of their existence, or of 

 the laws regulating the distribution of species from 

 their centres by migration, by transport, by ocean 

 currents, by elevations or depressions of the land, or 

 by any other causes at work under existing circum- 

 stances. So far as practical naturalists are con- 

 cerned, species are permanent within their narrow 

 limits of variation, and it would introduce an element 

 of infinite confusion and error if we were to regard 

 them in any other light. The origin of species by 

 descent with modification is as yet only a hypothesis. 

 During the whole period of recorded human observa- 

 tion not one single instance of the change of one 

 species into another has been detected ; and, singular 

 to say, in successive geological formations, although 

 new species are constantly appearing and there is 

 abundant evidence of progressive change, no single case 



