Chap. X. OF ORGANIC BEINGS. 297 



rate of change in terrestrial and in more higlily-org'anized pro- 

 ductions compared with marine and lower productions, by the 

 more complex relations of tlie hi<^her beings to tlieir organic 

 and inor^^nic conditions of life, as explained in a former chap- 

 ter. When many of the inhabitants of any area have become 

 modified and imjiroved, we can imderstand, on the principle of 

 competition, and from tlie all-important relations of organism 

 to organism in the struggle for life, that any form which does 

 not become in some deg'ree modified and improved, will be 

 liable to extermination. Ilencc we see why all the species in 

 the same region do at last, if we look to long-enougli intervals 

 of time, become niodilicd, 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 long'-enduring' for- 

 mations, 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 inter- 

 mittent intervals of time ; consequently the amount of organic 

 change exhibited by the fossils embedded in consecutive for- 

 mations is not equal. Each formation, on this view, does not 

 mark a new and complete act of creation, but only an occa- 

 sional scene, taken almost at hazard, in an ever slowly-chan- 

 ging 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 thougli the offspring 

 of one species might be ada])t{Hl (and no doubt this has occurred 

 in innunieral)le instances) to 1111 the place of another species' in 

 tlie economy of Nature, and thus supplant it; yet the two 

 forms — the- old and the new — would not be identically th.e 

 same ; for both would almost certainly inherit different char- 

 acters from their distinct progenitors, and organisms already 

 differing would vary in a different manner. For instance, it is 

 pist possible, if all our fantail jiigeons were destroyed, that 

 fanciers might make a new breed hardly distinguishable from 

 tlie present breed ; but if the parent rock-pigeon were likewise 

 destroyed, and imdcr Nature we have every reason to believe 

 that j»arent-fonns are generallv supplanted and exterminated 

 bv tlieir improved offspring, it is incredible that a fantail, iden- 

 tical with th(! existing breed, could he raised from any other 

 species of pigeon, or even from any other well-established race 



