FERNS, FOSSILS AND FUEL 
itself from sub-microscopic organisms through bacteria 
to algae were the seashores and perhaps the deltas of rivers 
and the edges of swamps and creeks. It is difficult to say 
whether salt or fresh water was first inhabited by living 
organisms. In any event, it was undoubtedly shallow 
waters that saw the origin of life, for there the conditions 
of light, temperature, and pressure would be more favor¬ 
able than at greater depths. Probably life originated in 
numerous places and gradually spread, filling all the 
coast lines, rivers, and swampy places, and from these 
crowding farther and farther into the highlands. 
In trying to visualize a picture of the vegetation of any 
given time during the Cambrian and Silurian periods 
of the earlier Paleozoic era, we should think of a coast¬ 
line whose waters are full of algae. There are green, 
brown, red, and blue water weeds; some mere floating 
ribbons, others twenty or thirty yards long and divided 
into many branches attached to rocks and floating in 
the clear warm water. This forest of water plants is 
inhabited by numerous sea-stars, sea-urchins, sea-ane¬ 
mones, jelly-fish, sponges, sea-lilies that had a plant-like 
appearance, but were animal bodies, sea-cucumbers, and 
corals of different colors, forming large reefs. Innu¬ 
merable shells are lying on the bottom of the sea, while 
others have been thrown up on the shore itself. Worms 
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