6 THE EVOLUTION OF MAN. 



enabled us to estimate and compare the relative durations 

 of the various periods of the earth's organic history. The 

 most direct standard for determining the relative duration 

 of geological periods is afforded by the thickness of the so- 

 called Neptunian strata or sedimentary rock, i.e., all those 

 strata which have been deposited, as mud, at the bottom 

 of the ocean, or under fresh water. These stratified sedi- 

 mentary rocks — limestone, clay, marl, sandstone, slate, etc. — 

 which constitute the great mass of mountain-chains, and 

 which are often several thousand feet in thickness, afford 

 us data for estimating the relative lengths of the various 

 periods of the earth's history. 



For the sake of completeness, I must say a few words as 

 to the development of the earth as a whole, briefly indicating 

 a few of the more prominent facts relating to this matter. 

 At the very outset we are confronted with the weighty 

 fact, that life originated on our planet at a certain definite 

 period. This is a proposition that is no longer gainsaid by 

 any competent geologist. We now know for certain that 

 organic life upon our planet actually began at a certain 

 time, and that it did not exist there from eternity, as 

 some have supposed. The indisputable proofs of this are 

 furnished, on the one hand, by physico-astronomical cos- 

 mogeny ; on the other, by the Ontogeny of organisms. Species 

 and tribes, like individuals, do not enjoy a perpetual 

 life. 127 They also had a beginning. The time which has 

 elapsed since the origin of life upon the earth up to the 

 present time (and with this period of time alone we are 

 here concerned) we call the " history of the organic earth," 

 as distinguished from the " history of the inorganic earth " 

 which embraces the period before the origin of organic life. 



