6 THE SMALLEST LIVING THINGS 



of living things with increasing thickness of the deposited por- 

 tions of the earth's crust. Relatively simple forms came first 

 and traces of their existence are to be found in the very earliest 

 rocks (pre-Cambrian) . In a general way this succession of 

 forms is shown in the table. 



Preservations in Rocks 



It must not be forgotten that soft-bodied animals had but 

 little chance of preservation in the rocks. Exceptional cases are 

 fossils of certain types of jellyfish which are found in Cambrian 

 deposits. Animals with hard parts, however, afford a fairly 

 complete history. Protozoa with limestone shells (Foramini- 

 fera) or silicious skeletons (Radiolaria) which abound in our 

 modern seas are well represented in the Lower Paleozoic and 

 Proterozoic eras. Similarly, the much more complex Brachio- 

 pods (lamp-shells), some types of which are living today, flour- 

 ished way back in the Cambrian period. One of them, Lingula, 

 is apparently the same today as it was in the Silurian period 

 millions of years ago. Representatives of our modern groups 

 of sponges, corals, Crustacea, mollusca, and spiders likewise first 

 appeared at that time. Animals with a cartilaginous skeleton 

 (sharks) first appeared in the Ordovician and Silurian periods; 

 amphibia in the Pennsylvanian; reptiles in the Mississippian 

 period; birds and first mammals in the Jurassic period; monkeys 

 and anthropoid (man-like) apes in the Tertiary period; and 

 the first men in the Quaternary period. 



Fossil plants, representative of the ferns, similarly, are found 

 as far back as the Devonian period, and flourished as great tree- 

 like forms in the Carboniferous period. Flowering plants ap- 

 peared in the Cretaceous period. 



Where and How Living Matter Originated 



This rapid survey is adequate to show that living things must 

 have appeared very early in the physical history of the earth. 

 Where and how living matter originated, while not entirely idle 

 speculation, are questions which have sometimes led to theories 

 which have not been confirmed by experiment. 



The creation of living things out of lifeless matter (spon- 

 taneous generation) has never been accomplished experimentally, 



