FOSSILS— THEIR NATURE AND INTERPRETATION. 8 1 



molecular replacement of the mineral of the original structure 

 by some other mineral, as calcite, silica, pyrite, etc. ; (3) the 

 cavity may be filled by detrital matter washed into the cavity 

 from outside. (D) The original substance may be changed in 

 molecular constitution, or even in chemical composition, 

 losing a part of its elements, or gaining other elements; thus, 

 a piece of wood may become coal, or a shell may become 

 crystalline calcite, or aragonite. (E) Finally, the fossil may 

 consist of traces left in the sediments while the animal was 

 alive, as footprints or other marks of organic activity. 



Fossils represent chiefly the Hard Parts of Organisms. — An 

 important generalization may here be made regarding all 

 fossils. Fossils represent organisms, but almost universally 

 they represent the ]iard parts of living organisms ; hence 

 the most valuable lessons to be learned from fossils must be 

 derived from the study of the hard parts of organisms. 

 These hard parts are the parts which have attained definite 

 and fixed form during the life development of the individual. 

 Soft parts, or organs, are adjustable to changing exterior con- 

 ditions, but its hard parts are already adjusted, and, there- 

 fore, they are an expression of the working adjustment of 

 the species, to the conditions of its environment, at the partic- 

 ular time in which it lived. 



Best and most perfectly adjusted Organisms of the Time left 

 their Records. — The history of organisms, which we particu- 

 larly trace in the study of fossils, is not the history of imper- 

 fect organisms struggling toward perfection, but it is the 

 history, for each age and epoch, of the perfected adjustment 

 of the organisms of the time to the particular conditions of 

 environment in which they lived. They did not die before 

 their time, overcome by the mythical fittest who are said to 

 survive in the struggle. They were the fittest, and died natu- 

 ral deaths, having provided before they gave up the struggle 

 for their progeny to succeed them. The hard parts record 

 the history of adults which had endured the struggle, and 

 thus represent the royal line of succession for the geological 

 ages. 



General Laws regarding the Occurrence of Fossils. — There 

 are certain general laws, concerning the occurrence of fossils 



