ANIMALS OF THE PAST 329 



psychological phenomena. These seem to have followed 

 one another in ascensive order." 



Though Chamberlin is willing, on the basis of observed 

 phenomena and mathematical calculations, to postulate 

 that life originated in the surface layers of the soil near 

 water, there is no actual record of such origin. The life of 

 the earliest geological periods has left no trace because the 

 rocks have all been "metamorphosed/' that is, changed by 

 pressure, heat, or other influences so as to destroy all 

 organic remains. Though Walcott has recently discovered 

 a few fossils in Pre-cambrian<rocks, the sedimentary depos- 

 its in the Cambrian are the first which contain abundant 

 animal remains. In the Cambrian most of the chief groups 

 of invertebrate animals appeared and many of them had 

 already attained a high degree of specialization during the 

 preceding formative ages. Fig. 113 gives a condensed 

 summary of the chief events since Pre-cambrian times. 

 The characteristic animals of the four great geological ages 

 will now be considered. 



PALAEOZOIC AGE 



Cambrian Period. The Cambrian fossils, so far as the 

 animals are concerned, are all of marine origin, and it is 

 doubtful if -terrestrial animals existed. In the oceans there 

 were protozoans, sponges, hydrozoans, jelly-fishes, corals, 

 round-worms, annelids, echinoderms (cystoids, starfishes, 

 sea cucumbers), brachiopods, clams, snails, cephalopods, 

 crustaceans and arachnids. Doubtless other types of inver- 

 tebrates were in existence but their remains have not yet 

 been found. Some of the species which existed in Cam- 

 brian oceans did not survive even until the next period, 

 and many whole groups became extinct during the Palae- 

 ozoic Age. Trilobites of various sizes were common. 

 The cephalopods usually had the body enclosed in a straight 

 or slightly curved shell. 



Ordovician Period. In the rocks from this period appear 

 the first remains of several classes of echinoderms (Bias- 



