22 EXTINCT MONSTERS 
THE IMPERFECTION OF THE RECORD. 
A very little consideration will serve to convince us that the 
record which Nature has kept in the stratified rocks is an in- 
complete one. There are many reasons why it must be so. It is 
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, eg. 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 
