How tlie History of the Past Is Read 



the distribution of rocks. Now, however, it is 

 recognized that fossils do not merely mark dif- 

 ferent epochs in the history of the past, but 

 that only by their aid can we determine the 

 relationships that animals bear to one another, 

 and only through them can we hope to trace 

 the development and distribution of living 

 things. 



The student of the past has at his command 

 the teeth and bones of vertebrates, sometimes 

 complete skeletons, their footprints, and, more 

 rarely, imprints of their coverings or even out- 

 lines of their forms. Of invertebrates, there 

 are shells or casts of shells, the hard coverings 

 of such creatures as crabs, impressions of soft 

 animals like jellyfishes, and the trails made by 

 these various creatures as they crept over the 

 shore. Fossilized logs and seeds, clean-cut im- 

 pressions of leaves, rushes, and seaweeds, com- 

 bine to tell the plant-life of the ancient world, 

 while the rocks in which all these are preserved 

 add their information to that of the fossils. 

 And with the aid of all this material it is pos- 

 sible to picture plant and animal life as it was 



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