OF ORQANIO BEINGS. 34,5 



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

 similar relations between the existing inhubitunts of dis- 

 tinct_ countries; for instance, the land-shells and coU-opter- 

 ous insects of Madeira have come to ditler considerably 

 from their nearest allies on the continent of Europe, 

 whereas the marine shells and birds have remained 

 unaltered. We can perhaps understand the a])parently 

 quicker rate of change in terrestrial and in more higlilv 

 organized productions compared with marine and lowe'r 

 productions, by the more complex relations of tlie liigher 

 beings to their organic and inorganic conditions of life, as 

 explained in a former chapter. When many of the inhab- 

 itants of any area have become modified and imj)roved, we 

 can understand, on the principle of competition, and from 

 the all-important relations of organism 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, per- 

 haps, be nearly the same; but as the accumulation of 

 enduring formations, rich in fossils, depends on great 

 masses of sediment being deposited on subsiding areas, our 

 formations have been almost necessarily accumulated at 

 wide and irregularly intei'mittent intervals of time; conse- 

 quently the amount of organic change exhibited by the fos- 

 sils embedded in consecutive 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 onre 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 supplant it; yet the two forms— the old and the 

 new— would not be identically the same; for both would 

 almost certainly inherit different characters from their dib- 



