CHAP. X.] PAL^ONTOLOGICAL COLLECTIONS. 253 



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. Unfor- 

 tunately we have no means f 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 Palceont oloyical 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. 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 enormously large pro- 

 portion 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 underlying 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 for ages in an unaltered 

 condition. The remains which do become embedded, if in sand 

 or gravel, will, when the beds are upraised, generally be dissolved 

 by the percolation of rain-water charged with carbonic acid. Some 

 of the many kinds of animals which live on the beach between 

 high and low water mark seem to be rarely preserved. For 

 instance, the several species of the Chthamalinae (a sub-family of 

 sessile cirripedes) coat the rocks all over the world in infinite 

 numbers: they are all strictly littoral, with the exception of a 

 single Mediterranean species, which inhabits deep water, and this 

 has been found fossil in Sicily, whereas not one other species has 

 hitherto been found in any tertiary formation: yet it is known 

 that the genus Chthamalus existed during the Chalk period. 

 Lastly, many great deposits requiring a vast length of time for 



