The Evidence for Evolution 
for the whole truth from them, for their memory is imperfect; 
and yet they can tell us a great number of important facts. 
From the time when the world was sufficiently cooled for 
water to condense on its surface, a continual process of unbuilding 
and rebuilding of rocks has gone on. Wind and water, heat and 
cold have laid their hands to the work, making sand and dust 
and gravel out of solid stone, and these products of their labours 
have been carried off to other places, laid down, and cemented 
together into new rocks. We do not know the exact age of any 
particular rock that has been made in this way, nor how long the 
process has been going on. At a rough guess it may be three or 
four hundreds of millions of years. The chronological succession 
of the different rock formations is, however, known, and their 
relative ages may be judged with considerable accuracy. Here 
and there, as time went on, the body of a plant or an animal 
was deposited in the sand or mud or chalk, and has remained 
in the resulting rocks, in the form of a fossil, through all the 
ages. If, then, we study the occurrence of fossils in this suc- 
cession of deposits, we ought to get some indications as to 
the inhabitants of the globe at various stages of its history. 
And if we do so, we meet unmistakable evidence that the 
lower and simpler types, both of animals and of plants, 
were in existence before the higher. Fig, 2 shows the facts 
with regard to the vertebrates, the great upper class of the 
animal kingdom. The first appearance of vertebrate fossils is 
in the Upper Silurian rocks, that is to say, somewhere after 
the middle of geological time. The fossils represent the 
lowest group of fishes. In the next great formation, the 
Devonian, fossils of two higher groups of fishes are to be found. 
The first land vertebrates, the amphibians, are doubtfully repre- 
sented in the upper or newer layers of the same formation, and 
definitely so in the next, the Carboniferous. Towards the end of 
the Carboniferous or early in the Permian epoch, the first reptiles 
appear, and in the following period, or after about three-fourths 
of geological time had passed, the earliest fossils of mammals 
occur. The significance of this sequence will become plainer when 
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