352 THE CAUSES OF THE xI 
composition. The jaw would thus be deposited 
immediately, while the rest of the body would 
float and drift away altogether, ultimately reaching 
the sea, and perhaps becoming destroyed. The 
jaw becomes covered up and preserved in the river 
silt, and thus it comes that we have such a 
curious circumstance as that of the lower jaws in 
the Stonesfield slates. So that, you see, faulty as 
these layers of stone in the earth’s crust are, 
defective as they necessarily are as a record, the 
account of contemporaneous vital phenomena 
presented by them is, by the necessity of the case, 
infinitely more defective and fragmentary. 
It was necessary that I should put all this very 
strongly before you, because, otherwise, you might 
have been led to think differently of the com- 
pleteness of our knowledge by the next facts I 
shall state to you. 
The researches of the last three-quarters of a 
century have, in truth, revealed a wonderful 
richness of organic life in those rocks. Certainly 
not fewer than thirty or forty thousand different 
species of fossils have been discovered. You have 
no more ground for doubting that these creatures 
really lived and died at or near the places in 
which we find them than you have for like 
scepticism about a shell on the sea-shore. The 
evidence is as good in the one case as in the other. 
Our next business is to look at the general 
character of these fossil remains, and it is a subject 
