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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