292 HISTORICAL GEOLOGY 



grown from very simple types, which were but single micro- 

 scopic cells of living substance. 



GROUPS OF ANIMALS AND PLANTS l 



The common forms of plants and animals. Every one 

 recognizes in a general way the difference between live things 

 and inanimate objects, and it is not difficult to perceive 

 certain fundamental traits which distinguish the one from 

 the other. We say the tree is alive because it grows by 

 taking into itself new substance, produces seed which makes 

 new trees, and finally dies. Granite cannot grow, in the same 

 sense, nor can it produce other granites; it is not alive. Sim- 

 ilarly, among living things themselves, we can usually dis- 

 tinguish at sight plants from animals, birds from fishes, 

 mosses from grasses, etc., but often without being able to 

 tell just what the differences are. The following descriptive 

 outline will help to make plain the distinctions between the 

 familiar groups of living things and will also impart some 

 acquaintance with others which are now rare or extinct, and 

 hence are unknown from everyday observation. 2 



Plants. Plants assimilate food, grow, and reproduce their 

 kind ; but most of them do not seem to feel, nor can they 

 move about at will. They have the ability to use as food, 

 not only water, but carbon dioxide from the air and certain 

 materials dissolved in the water. The vast majority of them 

 are green in color. There is scarcely any single feature, how- 

 ever, which will serve to distinguish all plants from animals. 



1 The following summary of the more important groups of living things 

 is inserted here to aid the large number of students of geology who have 

 little or no acquaintance with biologic science. It is necessarily very brief. 

 For additional information reference may be made to any of the more recent 

 textbooks of zoology and botany, or to the Zittel-Eastman Textbook of 

 Paleontology. 



2 Where possible the common English names for the different groups have 

 been used, but inasmuch as some of the divisions have only scientific 

 names, usuaHy of Latin or Greek origin, it has been necessary to make use 

 of them. 



