336 



STRUCTURE OF THE VERTEBRATES 



for the great strata and periods. IMore recently physicists have 

 used the degeneration of radio-active minerals as evidence re- 

 garding the time elapsed since particular periods were laid 

 down. All the evidence agrees that the earliest fossil bearing 

 rocks are billions of years old. Estimates vary widely, but com- 

 pared with the entire time involved a few million years is no 

 more than a day in the life of a geologist. 



Major Eras of Time 



These eras are arranged according to their relative age, the 

 most ancient at the bottom. The Cenozoic includes time up to 

 the present. Man evolved from the anthropoid stem in the late 

 Cenozoic, probably half a million years ago. Estimates as to 

 the age of the Archeozoic vary from one to several billion years. 



The Archeozoic and Proterozoic, which probably represent 

 more time than the other three combined, are only interesting 

 to the vertebrate anatomist in that they may in time give the 

 clue to the origin of the chordates. The vertebrates were not 

 autochthonous, but trace their history to the most ancient forms 

 of life. Evidences of their existence, however, begin in the 

 Palaeozoic era; no remains or impressions of the earlier soft 

 bodied chordates having been found. It is more than possible 

 that such remains were preserved, for a very small, cyclostome- 



