712 



THE ORIGIN OF SPECIES 



the rocks should provide a record of it in gradually changing 

 species from the older layers to the newer. To some extent his 

 prediction has been fulfilled ; we can trace a gradual increase in 

 size of the horses, and a progressive reduction in the number of 

 their toes (Fig. 546). In the elephants we can see the increase 

 of tusks and trunk, and reduction of molar teeth. In various groups 

 of molluscs, echinoderms and other invertebrates with an exo- 

 skeleton we can trace changes in the shape, size, and markings 

 of the shell (Fig. 547). Where the different types are found in 

 successive layers of the same rocks in the same place there can 

 be no reasonable doubt that those at the lower levels were the 

 ancestors of those above, and that specific and generic changes 

 have occurred. On a wider scale there is a danger of arguing in 

 a circle. The rocks all over the earth are arranged in a succession 

 to which a rough time-scale can be given (Table XI), but 

 correlation between one part of the w^orld and another is largely 



TABLE XI. 



