THE ARGUMENT FROM PALAEONTOLOGY 61 



bones, teeth, and footprints preserved. Mammals, 

 being terrestrial, are at a disadvantage. Birds, 

 whose bodies are light enough to float, are probably 

 devoured as food, and have less chance of preserva- 

 tion ; they are therefore rare as fossils. 



These parts can, as a rule, only be preserved in a 

 fragmentary condition. 



2. Only certain deposits can so preserve them, 

 notably mud. 



3. Animals must die at such places and times that 

 they can be preserved ; bones, &c, must be carried 

 down regularly to a particular locality. 



4. The deposit must extend over a very long time 

 and continuously, if the series is to be complete, or 

 even approximately so. 



5. The area must be in subsidence or else it would 

 be filled up. 



6. Fossils once imbedded must be raised above the 

 sea. 



7. They must escape denudation and be exposed 

 at some workable spot. 



8. The intervals between the deposits often 

 represent times of denudation. The denuded parts 

 will be the uppermost — i.e., the most recent, and will 

 break the series effectively by the removal and 

 destruction of fossil records. 



All these conditions can very seldom be fulfilled. 

 The difficulty is well illustrated by our state of 

 knowledge with regard to domestic animals. Where 

 are the bones of the intermediate forms between the 

 rock-pigeon and the pouter, or fantail, for example, 

 to be found ? Yet we know that these existed but 



