DISTRIBUTION IN TIME. 159 



enormous gaps in the palaeontological record, owing to the 

 different facilities with which different animals may be pre- 

 served as fossils. It is impossible here to enter at length 

 into this subject ; but there are obvious reasons why certain 

 groups of animals should never be found as fossils, or should 

 at best be but sparingly and imperfectly represented. 

 Thus, many animals are entirely soft-bodied, destitute of 

 hard parts capable of being preserved in a fossil condition, 

 and we can therefore never obtain evidence of the past 

 existence of such forms, though this affords no presump- 

 tion that they were non-existent at any given period. 

 Again, most sedimentary deposits have been laid down in 

 the sea, and contain, therefore, the remains of marine 

 animals, if not exclusively, at any rate in preponderating 

 numbers. Marine groups of animals are therefore much 

 more likely to be preserved than the inhabitants of lakes or 

 rivers. Lastly, almost all sedimentary accumulations have 

 been deposited in water, whether salt or fresh; and it follows 

 from this that the preservation of terrestrial or aerial animals 

 must, always have been of an accidental nature, so to speak, 

 depending upon the chance falling of such an animal into 

 water where sediment was being accumulated. It is only in 

 the rare cases in which an old land-surface has been pre- 

 served to us that we meet with the remains of such animals 

 as fossils properly belonging to the deposit in which they 

 occur. 



2. In the second place, as shown by the imperfection of 

 the geological record, there are vast periods in the earth's 

 history which are not known to us to be represented by 

 any deposits. This of necessity leads to our being totally 

 ignorant of the life of these same periods. As already 

 remarked, we can never expect wholly to fill up these 

 periods of "unrepresented time" by the discovery of new 

 deposits, and our palaeontological knowledge will therefore 

 ever remain more or less interrupted and incomplete. 



3. In the third place, we can seldom or never point to 



