340 ORIGIN OF SPECIES 



the most useful or beautiful animals, with no intention of 

 modifying the breed; but by this process of unconscious 

 selection, various breeds have been sensibly changed in the 

 course of two or three centuries. 



Species, however, probably change much more slowly, and 

 within the same country only a few change at the same time. 

 This slowness follows from all the inhabitants of the same 

 country being already so well adapted to each other, that new 

 places in the polity of nature do not occur until after long 

 intervals, due to the occurrence of physical changes of some 

 kind, or through the immigration of new forms. Moreover 

 variations or individual differences of the right nature, by 

 which some of the inhabitants might be better fitted to their 

 new places under the altered circumstances, would not always 

 occur at once. Unfortunately we have no means of deter- 

 mining, according to the standard of years, how long a 

 period it takes to modify a species; but to the subject of time 

 we must return. 



ON THE POORNESS OF PALJEONTOLOGICAL COLLECTIONS 



Now let us turn to our richest geological museums, and 

 what a paltry display we behold ! That our collections are 

 imperfect is admitted by every one. The remark of that ad- 

 mirable palaeontologist, Edward Forbes, should never be for- 

 gotten, namely, that very many fossil species are known and 

 named from single and often broken specimens, or from a 

 few specimens collected on some one spot. Only a small por- 

 "1^ tion of the surface of the earth has been geologically ex- 

 plored, and no part with sufficient care, as the important 

 discoveries made every year in Europe prove. No organism 

 ^ wholly soft can be preserved. Shells and bones decay and 

 disappear when left on the bottom of the sea, where sediment 

 is not accumulating. We probably take a quite erroneous 

 view, when we assume that sediment is being deposited over 

 nearly the whole bed of the sea, at a rate sufficiently quick 

 to embed and preserve fossil remains. Throughout an enor- 

 mously large proportion of the ocean, the bright blue tint of 

 the water bespeaks its purity. The many cases on record of 

 a formation conformably covered, after an immense interval 



