PAST HISTORY OF THE INVERTEBRATES 301 



the prey of the flesh-eaters. It stands to reason that the 

 first living things must have been able to provide for them- 

 selves and therefore must have been plants. As regards the 

 temperature at which life became possible, it is known that 

 plants live now in the hot springs of the West in water 

 reaching 180° F. 



There is very little direct evidence to indicate the charac- 

 ter of the life in the Archaean Era, but in all probability it 

 was of the simplest structure, even simpler than the single- 

 cell organisms of today. Although fossil remains of that 

 time are wanting, beds of limestone and graphite which were 

 formed then, point to the existence of life, for similar beds 

 formed in later eras are known to have been made through 

 the agency of organisms (polyps, mollusks, and the like, and 

 plants). 



The Age of Invertebrates. Following the Archaean Era, 

 several succeeding geological periods may be, for our pur- 

 pose, grouped under the general term, Age of Invertebrates, 

 from the predominance of these forms of life. The era was 

 very long, undoubtedly to be reckoned in millions of years. 

 We have no knowledge of the exact time that the different 

 species appeared, nor of the exact length of time each ex- 

 isted. Neither do we know from actual specimens all the 

 stages of evolution through which the early forms of life may 

 have passed in coming to the form and structure in which 

 we know their kin today. The intermediate types have been 

 lost on account of one catastrophe or another in the history 

 of the world. 



In the beginning of the era, life was marine, as far as fossil 

 records indicate. The earliest fossils are of sponges, corals, 

 sea lilies, worms, mollusks, and trilobites. Subsequently land 

 animals made their appearance in forms like the arachnids 

 and the insects. Before the close of the era a class of ver- 

 tebrate animals, the fishes, had come into existence. 



