THE GEOLOGIC FUNCTIONS OF LIFE. 665 



train carries the fundamental meaning. The dependent creatures 

 that follow the primary forms may be only incidentally, and perhaps 

 very slightly, adapted to the physical environment. 



(6) Adaptive relations. — Organisms depending on other organisms 

 for food or other necessary conditions of life, present many forms of 

 adaptation the better to secure their food and to use it. These adapta- 

 tions are the consequences and the signs of the assemblage, and are of 

 the greatest service in interpreting the place and significance of the 

 organisms in the assemblage. Teeth usually reveal the food of their 

 possessors, and hence teeth are among the most significant of fossils. 

 Fortunately their functions require them to be hard and durable, and 

 hence well suited to fossilization. The growth of low plants into trees 

 forced a notable series of adaptations in the animals that fed upon 

 them in the matter of height, of reaching members, of climbing, and 

 probably at length of parachuting and flying. In these and similar 

 ways the floras and faunas took on special phases because of the mutual 

 relations of their members. 



(c) Competitive relations. — The assembling of plants and animals, 

 with their prodigious possibilities of multiplication, brought competition, 

 and with it a struggle for food which often became a struggle for exist- 

 ence, and out of this grew innumerable modifications of form and habit. 

 These have become so familiar since the great awakening caused by 

 the doctrines of Darwin and Wallace that they need no elaboration here. 



(d) Offensive and defensive relations. — Within limits, plants are 

 benefited by the feeding of animals and respond by developing seeds 

 and fruits that especially invite such action, their compensation being 

 fomid in planting and distribution. It is ob^dous that, on the whole, the 

 continued growth of plants is largely dependent on the renc^^'al of a 

 supply of carbon dioxide through the agency of animals and somx plants, 

 bacteria in particular. Otherwise the supply would become so reduced 

 as to greatly limit plant hfe. It has been estimated^ that the whole 

 of the present supply of carbon dioxide would be consumed by plants 

 in one hundred years if the consumption continued at the present rate 

 and no carbon dioxide was returned. It is now well kno^^Ti that the 

 so-called decay by which carbon dioxide is freed is due more to micro- 

 scopic organisms than to inorganic processes. It seems clear, therefore, 

 that the continued acti\'ity of plants is largely due to their consump- 



^ S. W. Johnson, How Crops Feed, p. 47. 



