Seite - 340 - in The Origin of Species

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