366 ORIGIN OF SPECIES 



others; or, if changing, should change in a less degree. We 

 find similar relations between the existing inhabitants of dis- 

 tinct countries ; for instance, the land-shells and coleopterous 

 insects of Madeira have come to differ considerably from 

 their nearest allies on the continent of Europe, whereas the 

 marine shells and birds have remained unaltered. We can 

 perhaps understand the apparently quicker rate of change in 

 terrestrial and in more highly organised productions com- 

 pared with marine and lower productions, by the more com- 

 plex relations of the higher beings to their organic and in- 

 organic conditions of life, as explained in a former chapter. 

 When many of the inhabitants of any area have become 

 modified and improved, we can understand, on the principle 

 of competition, and from the all-important relations of or- 

 ganism to organism in the struggle for life, that any form 

 which did not become in some degree modified and improved, 

 would be liable to extermination. Hence we see why all 

 the species in the same region do at last, if we look to long 

 enough intervals of time, become modified, for otherwise 

 they would become extinct. 



In members of the same class the average amount of 

 change during long and equal periods of time, may, perhaps, 

 be nearly the same; but as the accumulation of enduring 

 formation, rich in fossils, depends on great masses of sedi- 

 ment being deposited on subsiding areas, our formations have 

 been almost necessarily accumulated at wide and irregularly 

 intermittent intervals of time; consequently the amount of 

 organic change exhibited by the fossils embedded in consecu- 

 tive formations is not equal. Each formation, on this view, 

 does not mark a new and complete act of creation, but only 

 an occasional scene, taken almost at hazard in an ever 

 slowly changing drama. 



We can clearly understand why a species when once lost 

 should never reappear, even if the very same conditions of 

 life, organic and inorganic, should recur. For though the 

 offspring of one species might be adapted (and no doubt 

 this has occurred in innumerable instances) to fill the place 

 of another species in the economy of nature, and thus sup- 

 plant it; yet the two forms — the old and the new — would 

 not be identically the same; for both would almost certainly 



