THE LAPSE OF TIME. 819 



fairer^ with the effects which follow from unconscious 

 selection, that is, the preservation of the most useful or 

 beautiful animals, with no intention of modifyino- 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 slowlv, 

 and within the same country only a few change at tlie sanie 

 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 tlie 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 

 circumstance, would not alwavs occur at once. Un- 

 fortunately 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 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 palaeontologist, 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 eartli 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 sediment is not accumulating. AVe prob- 

 ably take a quite erroneous view, when we assume tluit 

 sediment is being deposited over nearly the whole bed of 

 the sea, at a rate sufficiently quick to imbed and preserve 

 fossil remains. Throughout an enormously large propor- 

 tion of the ocean, the bright blue tint of the water be- 



