Decembeb 31, 1009] 



SCIENCE 



943 



but they are scarcely less real in theoretic 

 passibility. 



The well-being of life is hemmed in be- 

 tween a suitable proportion of moisture in 

 the air dependent on a competent area of 

 water-surface to supply it, on the one hand, 

 and a diluvial excess of water, on the 

 other. Universal deluges and universal 

 deserts would alike be disastrous. A few 

 thousand feet more of water-depth or a few 

 thousand feet less would alike seriously 

 restrict the class of life to which we belong. 



In even a more serioas way the habita- 

 bility of the earth is conditioned on a nar- 

 row range of mean temperature— a range, 

 roundly speaking, of 100° Centigrade. 

 This is scarcely 5 per cent, of the range of 

 natural temperatures on the earth and a 

 still smaller per cent, of the range of tem- 

 peratures in the heavens. A few miles 

 above us and a few miles below us, fatal 

 temperatures prevail. It is profoundly 

 significant that the thermal states of the 

 narrow zone of life on the face of the earth 

 should have been kept within so close vari- 

 ations as to permit the millions of species 

 forming the great genealogical lines lead- 

 ing up from the primitive tji^es to have 

 perpetuated their lineages in imbroken 

 continuity for such ages, while the preva- 

 lent temperatures a few miles above them 

 or a few miles below them, as well as in 

 space generally, would have been fatal. 

 While the necessary heat is dependent on 

 the sun, this control of temperature seems 

 to have been intimately related to the at- 

 mo.sphere and is a further inde.x of its 

 specially critical functions. 



To appreciate the full significance of the 

 control of life conditions within these nar- 

 row limits when the possibilities were so 

 free and so wide, there Ls need for some 

 tangible index of the time, but there are at 

 present no means for the close measure of 

 the geologic ages, merely rough estimates 



of the order of magnitude. Life was far 

 advanced when a readable record first be- 

 gan to be made ; but yet since that record 

 began, at least 100,000 feet of sediments— 

 not to choose the largest estimates— have 

 been laid down by the .slow methods of 

 wash from the land and lodgment in the 

 basins. The estimate of the years thus 

 represented has been put variously from 

 50,000,000 to 100,000,000, with indeed 

 higher figures as well as lower. Merely to 

 roughly scale the order of magnitude, and 

 without pretease of accuracy, let us take 

 the midway figure of 75,000,000 years as 

 representative. Let this be divided into 

 fifteen periods of 5,000,000 years each and 

 these will roughly represent the technical 

 "periods" of geologists. By this rough 

 scale we may space out such of the great 

 events as we need now note. 



Slight and changeable excesses of evapo- 

 ration over precipitation and the reverse 

 prevail widely, but only intense and per- 

 sistent aridity gives rise to thick deposits 

 of salt, gypsum and other evaporation 

 products over large areas— with perhaps 

 some exceptions— for in nearly all large 

 natural basins the area that collects rain- 

 fall is notably larger than the closed basin 

 within it that alone can retain water for 

 continuous evaporation. It is therefore 

 fairly safe to infer clear .skies and pro- 

 nounced aridity when beds of salt and 

 gypsum occur over large areas, especially 

 if accompanied by appropriate physical 

 characters and by such types of life only 

 as tolerate high salinity, or show pauper- 

 ization, or by a total absence of life. 



Now extensive deposits of salt and gyp- 

 sum are found in the Salt Range of India, 

 in strata of the Cambrian period, the 

 earliest of the fifteen that make up our 

 rough scale of 75,000,000 years. Because 

 these lie .so near the beginning of the geo- 

 logic record they afford a singularly in- 



