666 GEOLOGY. 



tion by animals and other plants. But still, though the larger good of 

 plants is conserved by the predaceous action of animals, and of certain 

 parasitic and saprophytic plants, their individual preservation is often 

 conserved by defensive devices, such as thorns, poisons, bitter com- 

 pounds, etc. This is notably true in desert regions where the condi- 

 tions are hard and the total extinction of plants would be threatened 

 if animals were permitted to feed freely upon them. Within the animal 

 world, the preying of one form upon another is the main source of that 

 great struggle for existence which has characterized the whole known 

 history of life, and has been one of the influential factors in shaping the 

 evolution of life and in modifying the special aspects assumed by the 

 floras and faunas of each period. 



Implied forms of life. — The full meaning of the fossils of any period 

 can only be gathered by duly considering these relationships in their 

 interpretation. The existence of animals implies the existence of plants 

 in supporting abundance, whether the record contains their relics or not; 

 an animal with a protective covering implies an enemy; a tooth of a 

 specific kind impHes the appropriate class of food, etc. While infer- 

 ences of this kind are subject to error, they are at present the only 

 means by which the faunas and floras of most ages can be rounded out 

 into a rational assemblage of organisms, that is, an assemblage that 

 affords the necessary food for its members and an adequate function 

 for the offensive and defensive devices which its members present. 

 Only a small part of the life that lived Avas fossilized, and only a small 

 part of the fossils actually carried in the strata have been collected, 

 because only a small part of the strata are exposed at the surface. The 

 direct record now accessible is, therefore, very incomplete and hence 

 the need — and in the need the excuse — for adding the forms that are 

 implied by the character of the known fossils. 



(2) Assemblages Influenced by Environment. 



It has been noted that some animals depend for existence on other 

 animals; that ultimately all animals depend on plants, and that green 

 plants alone can make food directly from inorganic material. Green 

 plants, therefore, head the train of dependencies, and their relations 

 to the physical conditions that surround them are the primal relations. 



