. 448.] HEREDirV 



we shall see that, in spite of their diversity and ( 

 may bring them into an orderly classification. ^ 

 them under these four headings— i, unchan^m 

 occasional, 4, periodic. Among unchanging fact 

 may be mentioned,' viz. the atmosphere, \vat< 

 gravity, the earth as a whole. Though these ;i 

 unchanging influences I shall discuss only these 

 The earth's atmosphere, consisting of nitro: 

 oxygen (20^^), with only a small fraction of i 

 o.xide and other gaseous matters, possesses unc i 

 properties and exerts a pressure which varies 01 

 taking only short intervals of time for the comi):) 

 not at all, taking long intervals of time. P^'r e> 

 metric pressures in any given locality are the .si 

 tury as in the last, and though we may ha\ c " 1< 

 barometers at different times, these variations 

 The buoyancy, diversity, color, transparency, p 

 of the earth's atmosphere are the same to-da)- 

 years ago, as far as we know, and though these 

 pressure of the atmosphere may change and m: 

 during the lapse of the millions of years di ■- 

 is likely to exist and has existe( 

 ing so far as millions of generations ot liviii 

 far as millions of series of lifeless things, ar. 



The composition of the earth's atin(>si.)h 

 little, if at all. The proportion of carbon-di 

 greater in an earher time, but tv/i ..n. 

 tions have been what they are now. I^et i 

 that the proportions of nitrogen, o.\\,^^'n. ^i'^' 

 changed somewhat, taking the whole atmosj) 

 a small part of it merely. If only the propo 

 there would still have been nitrogen, ox\gc 

 in the air, and these gases themsches are ui 

 not imagine some oxygen being any ditterc 

 if we have carbon dioxide we have nece>sai 

 the same composition and pro]K'Un-> s 

 The same of the inert gas nitrogen. 



lich the earth 



