THE DISTRIBUTION OF ANIMAL LIFE. 93 



stratified rocks, because they are laid down layer 

 after layer, or aqueous rocks, because they are formed 

 under the agency of water. The action of the 

 weather continually wears away, or, in the phrase of 

 the geologist, denudes, the surface of all land or rock 

 that lies exposed to the air ; and the bits thus re- 

 moved, often so fine as to be imperceptible, are 

 washed away by rain, and carried down to the sea 

 by rivers, and finally deposited in layers at the bottom 

 of the sea, the layers consisting of shingle, sand, or 

 mud, according to the fineness to which the particles 

 have been ground down during their removal and 

 transit. The dead remains of animal and vegetable 

 forms sink and get mixed with these deposits ; or, 

 when the deposit is laid down deep in the ocean, far 

 from the land and the silt that rivers bring down 

 from it, the deposit may even consist almost entirely 

 of organic remains. The majority of fossiliferous rocks 

 are formed in this way, the cases in which local causes 

 give rise to the formation of local deposits in places 

 above the level of the sea being comparatively rare. 



Following out the logical consequences of the 

 Darwinian theories, we should expect to find in the 

 oldest rocks remains only of the simplest animals, 

 and to meet with a gradually ascending series, in 

 which the more highly organized animals appeared 

 gradually. Practically this is what we do find. But 

 the geological record is for many reasons very im- 

 perfect. The most obvious of these reasons is, that 

 new stratified rocks are not formed all over tlie globej 



