270 THE POORNESS OF OUR (.Chap. *• 



posed 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 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 slow* 

 ness 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 occur- 

 rence 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 

 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 the Poorness of our Pakcontological 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 palaeonto- 

 logist, 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 im- 

 portant discoveries made every year in Europe prove. 2STo organism 

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

 when left on the bottom of the sea, where sediment is not accumu- 

 lating. 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 bespeaks its purity. The many cases on 

 record of a formation conformably covered, after an immense 

 interval of time, by another and later formation, without the under- 

 lying bed having suffered in the interval any wear and tear, seem 

 explicable only on the view of the bottom of the sea not rarely lying 



