i6 



HUMAN BIOLOGY 



perature, is hard to see. The stratosphere, or isothermal 

 layer, in the upper part of the Earth's atmosphere, is almost 

 free from water vapor, though the troposphere, or lower 

 atmosphere, is full of it. If a similar demarcation exists on 

 Venus, and if the lower atmosphere is full of clouds to its 

 very top, the observed phenomena may perhaps be 

 exphcable. 



The absence of oxygen cannot be similarly explained, 

 for, not being subject to condensation, it would diffuse 

 freely into the upper atmosphere. It must apparently be 

 accepted that Venus is devoid of oxygen, and this points 

 strongly to the conclusion that neither animal nor vegetable 

 hfe is present upon the planet. Why this should be the 

 case is a matter of conjecture. The suggestion has recently 

 been made by Webster that, if the planet's rotation is 

 slow, and the difference of day and night temperatures as 

 great as is observed, winds of great violence will blow 

 from the warmer to the colder regions, and it may well 

 be that marine erosion has more than overtaken the forces 

 of elevation, so that the planet has been, for the most of 

 its history, covered by a shoreless sea, in which life had 

 no chance to arise; nor would its opportunity have been 

 much better on incessantly storm-swept coasts. It may be, 

 on the other hand, that there is no water on Venus, in 

 spite of the a priori probability, and in this case the absence 

 of life follows necessarily. 



Though much remains to be known about the conditions 

 which prevail upon this, the nearest of the planets, the 

 conclusion that life, as we know it, is not to be found on 

 Venus appears to rest on firm foundations. 



So we come finally to Mars, and to a situation singularly 

 contrasting with that of Venus. Though of little more 

 than half the Earth's diameter. Mars has a surface-gravity 

 38 per cent as great as the Earth's, and is fully able to 

 retain an atmosphere. It actually possesses one, as many 

 things prove, but one less extensive than ours. The amount 

 of atmosphere above a square mile of the surface appears 

 to be between one-tenth and one-half as much as on the 

 Earth, which, under the smaller force of gravity, would give 

 an atmospheric pressure at the surface lying between 



