554 THE CHANGING WORLD 



These estimates have been arrived at by various methods. That 

 of Sollas, for instance, is based upon observed rates of erosion and 

 sedimentation, although such rates are known to vary considerably 

 with the conditions involved. Barrell's computations, on the other 

 hand, depend upon the transformation of radio-active substances in 

 the earth's crust. This latter method is probably the most reliable 

 criterion for measuring the passage of time, for the reason that it has 

 been experimentally demonstrated that the rate at which the trans- 

 formation of radio-active substances occurs is constant. Thus, it 

 serves as a reliable time-meter for determining the age of the rocks 

 in which these substances are found. Uranium salts, for example, 

 by discharging three helium atoms, become transformed into radium, 

 which, in turn, undergoes still further progressive change, accom- 

 panied by the release of energy, by shooting off five more helium 

 atoms when it finally becomes stable in the form of inert lead. Conse- 

 quently, since this accurately timable transformation takes place 

 at a definite rate, the time of the laying down of a stratum of rock 

 in the earth's crust containing uranium-lead, or other radio-active 

 elements in various stages of transformation, can be dated with 

 considerable accuracy. 



It will be seen from the table that an average of the opinions of the 

 twelve experts cited indicates that the lapse of time during the 

 Cenozoic era covered 10.64 per cent of the time since the first known 

 plants and animals lived. Furthermore, the Cenozoic era is sub- 

 divided into periods of varying duration, of which the last, or 

 Pleistocene period, meaning "most recent," is estimated to be 

 approximately one sixth of the entire Cenozoic era, or, according 

 to a most conservative guess, about 500,000 years. This is the 

 spacious stretch of time in which we are to hunt for our earliest 

 human ancestors. Since our primate cousins are known from their 

 fossil remains to have existed as far back as the Oligocene period, 

 there is no occasion to apologize for, or to feel in any way embarrassed 

 by, the grotesque character of relatives so remote. 



Aside from the indirect testimony of comparative anatomy and 

 embryology, based upon the probable time needed to evolve so com- 

 plex an organism as man, there are two lines of indisputable evidence 

 of the great antiquity of mankind. The first deals with artifacts, 

 or the tools and weapons considered in a previous section, which could 

 only have been fashioned by human hands, and the second, with the 

 occurrence of human fossils, the "poor Yoricks" that have frequently 



