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, ¢.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 
