CHAP. 1.] INTRODUCTION. Teal 
We must remember, however, that the.rocks of 
the Silurian system, overlaid by ten miles’ thickness 
of sediment é¢ntombing a hundred successive faune, 
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 
