THE IMPERFECTION OF THE GEOLOGICAL RECORD 275 



animals, that they have formed what well deserves to be called 

 a new sub-breed. Few men have attended with due care to any 

 one strain for more than half a century, so that a hundred years 

 represents the work of two breeders in succession. It is not to be 

 supposed that species in a state of nature ever change so quickly 

 as domestic animals under the guidance of methodical selection. 

 The comparison would be in every way fairer with the effects 

 which follow from unconscious selection, that is, the preservation 

 of the most useful or beautiful animals, with no intention of 

 modifying the breed; but by this process of unconscious selec- 

 tion, 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 coun- 

 try 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 cir- 

 cumstances, would not always occur at once. Unfortunately we 

 have no means of determining, 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 TKE POORNESS OF PAL^ONTOLOGICAL 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 admirable palaeon- 

 tologist, Edward Forbes, should never be forgotten, 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 portion of the surface of the earth 

 has been geologically explored, 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 sedi- 

 ment 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 enormously 

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



