EVOLUTION 



351 



tests to be closely related are also known by morphological similarities 

 to have had recent common ancestry. 



Extinct Animals. — The general relations of fossil animals to each 

 other and to the geological formations are explained by evolution. The 

 fact that, as one passes up through the stratified rocks, the animal re- 

 mains gradually change, is precisely what evolution would require. The 

 occurrence of fossils similar to hving animals, but differing from them in 

 some features, is no longer puzzling if evolu- 

 tion be assumed. Thus, as was pointed out in 

 the preceding chapter, the early mastodons had 

 long protruding faces, while the modern animals 

 most resembhng them, the elephants, have the 

 front of the skull much flattened. Fortunately 

 for the explanation of this difference, the later 

 fossil mastodons had much shorter skulls than 

 did the earlier ones. If the later forms evolved 

 from the earlier ones, and if the shortening were 

 continued down to the present time, there is no 

 difficulty in accounting for the ev^ident similarity 

 of the mastodons and elephants in most respects. 

 The differences in the teeth and lower jaw and 

 proboscis and height of skull are explained in 

 the same way. The mastodons and elephants 

 are so much alike in general, that one is practi- 

 cally forced to regard them as members of the 

 same fine of descent. But if they are thus 

 related, the form of the skull and the shape and 

 number of the teeth and other features have 

 been altered. The occurrence of horse-like 

 animals in successive, geological periods presents 

 precisely the same problem. If the fossils of 

 these animals be arranged in chronological 

 order, there is so little difference between any 

 one animal and those which immediately 

 precede and follow it, that it is difficult to 

 avoid the assumption that they were related to 

 one another as ancestor and descendant. But if this be done, we must 

 assume evolution. 



Geographical Distribution. — Many phenomena of distribution 

 which would be meaningless without the doctrine of evolution are entirely 

 natural if modification occurred during descent. Along with the evolu- 

 tion of animals,it is often necessary to assume an evolution of the earth's 

 surface, if the facts of animal distribution are to be made intelUgible. 

 Thus, the marsupials (pouched animals, including the opossum and kan- 



FiG. 242.— Adult free-liv- 

 ing barnacle of the genus 

 Lepas, with half of its shell 

 removed. The jointed ap- 

 pendages and some other 

 features make it an arthro- 

 pod. Its larva resembles 

 that of Sacculina, Fig. 241, .4. 

 Compare the complex adult 

 of Lepas with the degenerate 

 adult of the parasitic Sac- 

 culina, Fig. 241, C. 



