NATURAL SELECTION 



during an immense but unknown period the surface of the 

 earth has undergone successive changes ; land has sunk be- 

 neath the ocean, while fresh land has risen up from it ; 

 mountain chains have been elevated ; islands have been 

 formed into continents, and continents submerged till they 

 have become islands ; and these changes have taken place, 

 not once merely, but perhaps hundreds, perhaps thousands of 

 times. That all these operations have been more or less 

 continuous but unequal in their progress, and during the 

 whole series the organic life of the earth has undergone a 

 corresponding alteration. This alteration also has been 

 gradual, but complete ; after a certain interval not a single 

 species existing which had lived at the commencement of the 

 period. This complete renewal of the forms of life also 

 appears to have occurred several times. That from the last 

 of the geological epochs to the present or historical epoch, 

 the change of organic life has been gradual : the first appear- 

 ance of animals now existing can in many cases be traced, 

 their numbers gradually increasing in the more recent forma- 

 tions, while other species continually die out and disappear, 

 so that the present condition of the organic world is clearly 

 derived by a natural process of gradual extinction and crea- 

 tion of species from that of the latest geological periods. 

 We may therefore safely infer a like gradation and natural 

 sequence from one geological epoch to another. 



Now, taking this as a fair statement of the results of 

 geological inquiry, we see that the present geographical dis- 

 tribution of life upon the earth must be the result of all the 

 previous changes, both of the surface of the earth itself and 

 of its inhabitants. Many causes, no doubt, have operated of 

 which we must ever remain in ignorance, and we may, there- 

 fore, expect to find many details very difficult of explanation, 

 and in attempting to give one, must allow ourselves to call 

 into our service geological changes which it is highly probable 

 may have occurred, though we have no direct evidence of 

 their individual operation. 



The great increase of our knowledge within the last twenty 

 years, both of the present and past history of the organic 

 world, has accumulated a body of facts which should afford 

 a sufficient foundation for a comprehensive law embracing and 



