CHAPTEK III 



THE ERA OF INVERTEBRATES 



HoTv, and when, and where life first ap- 

 peared in this world of ours we do not know * — 

 in all ^probability we shall never know — but we 

 may be sure that it began with extremely small 

 and simple forms, and it is fairly certain that 

 these originated in the sea at a time long before 

 any of the animals whose remains have actually 

 been preserved had made their aj^pearance. 

 Theoretically, the further back we go in time 

 the simpler should we find the life of that 

 period, and the earliest animals of all should 

 be the simplest in structure. Up to a certain 

 point facts accord with theory, and as we go 

 down in the rocks the higher animals drop out 

 one by one, mammals and birds disappearing 



* In this connection it would be well to read The Origin of 

 the Oldest Fossils, and the Discovery of the Bottom of the Sea, by- 

 Prof. W. K. Brooks. Among other places it has been reprinted 

 in the Report of the Smithsonian Institution for 1894. 



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