314 THE LAPSE OF TIME, 



than they differ from each other. Hence, in all such 

 cases, we should be unable to recognize the j^arent-form of 

 any two or more species, even if we closely compared the 

 structure of the parent with that of its modified descend- 

 ants, unless at the same time we had a nearly perfect chain 

 of the intermediate links. 



It is just possible, by the theory, that one of two 

 living forms might have descended from the other; for 

 instance, a horse from a tapir ; and in this case direct 

 intermediate links will have existed between them. But 

 such a case would imply that one form had remained. 

 for a very long period unaltered, while its descend- 

 ants had undergone a vast amount of change; and the 

 principle of competition between organism and organism, 

 between child and parent, will render this a very rare 

 event; for in all cases the new and improved forms of life 

 tend to snpplant the old and unimproved forms. 



By the theory of natural selection all living species have 

 been connected with the parent-species of each genus, by 

 differences not greater than we see between the natural and 

 domestic varieties of the same species at the present day; 

 and these parent-species, now generally extinct, have in 

 their turn been similarly connected with more ancient 

 forms; and so on backward, always converging to the 

 common ancestor of each great class. So that the number 

 of intermediate and transitional links, between all living 

 and extinct species, must have been inconceivably great. 

 But assuredly, if this theory be true, such have lived upon 

 the earth. 



Oif THE LAPSE OK TIME, AS INFERRED FROM THE RATE 

 OF DEPOSITIOi^ AND EXTENT OF DENUDATION. 



Independently of our not finding fossil remains of such 

 infinitely numerous connecting links, it may be objected 

 that time cannot have sufficed for so great an amount of 

 organic change, all changes having been effected slowly. 

 It is liardly possible for me to recall to the reader who is 

 not a practical geologist, the facts leading the mind feebly 

 to comprehend the (apse of time. He who can read Sir 

 Charles LyelTs grand work on tlie Principles of Geology, 

 which the future historian will recognize as having pro- 



