EVIDENCES— GEOLOGY 129 



environment. When fossils possess similar structures we can be 

 certain that they met similar needs, and that the environment of 

 the extinct animal was therefore similar to that of the one now 

 living. Plants are very closely linked with the physical environ- 

 ment, hence fossil plants are valuable evidence of the temperature 

 and rainfall of the period to which they belong (Fig. G9). The 

 teeth of an animal are equally indicative of the kind of food which 

 it eats, and if a fossil has shearing teeth, we know at once that it 

 was carnivorous, while broad, grinding teeth indicate grazing 

 forms. Thus aridity, forestation, temperature, and various haljits 

 are disclosed by these remains, and a careful study of all available 

 evidences has pieced out the record of prehistoric life to a remark- 

 able degree. 



Succession of Forms. The general illustration of evolution de- 

 rived from such sources is adequately expressed in the geological 

 table. If evolution has occurred, the most primitive creatures 

 would necessarily have existed in the earliest periods of the proc- 

 ess, and successive degrees of complexity would become evident 

 as time passed. This proves true to such a high degree that we 

 are justified in looking upon it as a further proof of evolution, and 

 in adding to the chronological succession the earliest stages, which 

 can only be inferred, since the organisms must have been too small 

 and too delicate to be preserved readily as fossils. Fossils have 

 been reported even from the Archeozoic, but they are open to 

 doubt. With this initial step alone based only upon estimate, we 

 find that the Proterozoic rocks contain only primitive inverte- 

 brates, and that a gradual ascension of forms proceeds through the 

 remaining eras. Just as we see in the species now living the order 

 proceeds through the higher invertebrates to the fishes, the most 

 primitive of vertebrates, thence through the Amphi!)ia and the 

 reptiles to the birds and mammals, and finally man. Since ani- 

 mals are entirely dependent on the green plants for their food 

 supply, it is significant that in this record the first known land 

 floras precede the true terrestrial animals in development, and that 

 in many other known details the various parts of the record coincide. 



By reference to the table, it will be seen that the late Protero- 

 zoic, while it is evidently the period during which the marine 

 invertel^rates were the dominant form of life, is characterized by a 

 scarcity of fossils. For this reason it is impossii)le to judge the 

 phylogenetic association of species which must have existed. 



