Evolution 



the differences and likenesses of these various groups are explained. 

 Each of these great groups in turn formed the dominant animal 

 population of the globe, and each in turn was superseded, although 

 not entirely, by the next. The mammal group itself appears 

 to be on the wane, overcome in the struggle for dominance by 

 its own latest and most remarkable member, man himself. 



The broad facts in the history of hving things upon the earth 

 are, then, in accordance with the theory of Evolution. The chain 

 of types is indeed a broken one, the gaps being many, and some 

 of them wide. But this is readily to be understood from the com- 

 parative scarcity of fossils, and the imperfection of the geological 

 record. 



In certain particular instances, however, very complete series of 

 fossil forms have been discovered, connecting, by small grada- 

 tions, modern animals with greatly different extinct types. One 

 of the most complete of such series has been discovered for 

 the horse. The changes that have occurred in the evolution of 

 this animal have been mainly in three directions — increase in 

 size, reduction in the number of toes from the original five to 

 the final one, and deepening of the crowns of the teeth, so as to 

 render them capable of longer wear. From the Eohippus of 

 early tertiary times, an animal of about the size of a fox terrier, 

 with five toes behind, and four with the vestige of the fifth in 

 front, there is a complete connecting series reaching up to the 

 modern horse, with its single remaining toe and the vestiges of two 

 others. A few of the main links in this chain are illustrated in 

 Fig. 3. It is impossible to regard such a series without having 

 the idea of Evolution strongly suggested to the mind. 



In the second place, there is evidence for Evolution in the 



fact that marked changes can and do occur in the characters of 



living races of organisms. There is ample evidence, for example, 



that all our modern breeds of pigeons are descended from the wild 



rock-dove. How markedly some of these differ from their wild 



ancestor, and among themselves, may be seen from Fig. 4. 



The size of some is twice as great as that of others. The bill in 



some is greatly increased in length, is almost ludicrously reduced 



8 



