20 EXTINCT MONSTERS. 



not to be expected that these rocks should contain anything 

 like a complete collection of the remains of the various tribes 

 of plants and animals that from time to time have flourished in 

 seas, lakes, and estuaries, or on islands and continents of the 

 world. In endeavouring to trace the course of life on the globe 

 at successive periods, we are continually met by want of evidence 

 due to the " imperfection of the record " — to use Darwin's phrase. 

 The reasons are not far to seek. The preservation of organic 

 remains, or even of impressions thereof, in sedimentary strata is, 

 to some extent, a matter of chance. It is obvious that no wholly 

 soft creature, such as a jelly-fish, can be preserved ; although on 

 some strata they have left impressions telling of their existence 

 at a very early period. 



A creature, to become fossilised, must possess some hard part, 

 such as a shell, e.g. an oyster (fossil oysters abound in some 

 strata) ; or a hard chitinous covering, like that of the shrimp, or 

 the trilobites of Silurian times ; or a skeleton, such as all the 

 backboned (vertebrate) animals possess. 



But even creatures that had skeletons have not by any means 

 always been preserved. Bones, when left on the bottom of the 

 sea, where no sediment, or very little, is forming, will decay, and 

 so disappear altogether. As Darwin points out, we are in error 

 in supposing that over the greater part of the ocean-bed of the 

 present day sediment is deposited fast enough to seal up organic 

 remains before they can decay. Over a large part of the ocean- 

 bed such cannot be the case ; and this conclusion has, of late 

 years, been confirmed by the observations made during the 

 fruitful voyage of H.M.S. Challenger in the Atlantic and Pacific 

 Oceans. 



Again, even in shallower parts of the old seas, where sand or 

 mud was once deposited, fossilisation was somewhat accidental ; for 

 some materials, being porous, allow of the percolation of water, 

 and in this way shells, bones, etc., have been dissolved and lost. 

 Thus sandstone strata are always barren in fossils compared to 



