INTRODUCTION 39 



without going outside the range of the fish-class, others 

 were so appreciably changed as to become salamanders. 

 Geology shows, in fact, that at a certain period sala- 

 manders were the only vertebrates on the earth beside 

 the fishes. We do not find these at the earlier stages ; 

 they can only have been evolved from the fishes, as 

 these are the nearest to them of all the animals that 

 lived at the time, and the structure of the salamander 

 approaches so closely to that of the fish that we can 

 conceive the period as sufficient for the transformation 

 a transformation which is far less considerable than the 

 conversion of a worm into a salamander. 



From these salamanders the actual salamanders and 

 frogs must have descended on the one hand, and 

 the reptiles, leading on to the birds and mammals, on 

 the other. We may form a picture of the transformation 

 of animals with the figure of the tree. At a certain 

 period a side-branch, the fishes, grew out of the trunk ; 

 the branch grew on, and put forth another side-branch, 

 the salamanders, which in turn sent out branches. Thus 

 we can compare the growth of the organic world with 

 the growth of a tree. At first there was a single trunk, 

 the simplest organisms. Branches grew out from the 

 trunk, and in turn produced twigs, until a mighty tree 

 arose with many branches and innumerable twigs. 



The view that the animal kingdom was developed in 

 this way from the simplest forms is called " the theory 

 of evolution." 1 It is now generally accepted ; there are 



1 Also the theory of descent, or transformism. We must carefully 

 distinguish between : i, the statement that living organisms have been 

 developed from other forms , and 2, the theory as to how they were 

 developed, and by what forces. The first is the theory of evolution, the 



