MODIFICATION OF THEORY EXAMINED. 93 



produce a glacial epoch, irrespective of the necessary 

 physical conditions. We know with certainty that 

 those physical conditions which, according to the 

 theory, were the direct cause of the glacial epoch on 

 our globe, cannot possibly exist on the planet Mars. * 

 Just take one example : either the properties of water 

 on the planet Mars or the conditions of its atmosphere 

 must be totally different from that of our earth ; for 

 were our earth removed to Mars' distance from the 

 sun, our seas would soon become solid ice, and we could 

 have neither snow nor rain, ocean-currents, nor any 

 of the necessary conditions for secular change of 

 climate. This is doubtless not the present state of 

 Mars ; but the reason of this can only be that the 

 physical and meteorological conditions of the planet 

 must be wholly different from those of the earth. 



When we reflect that a very slight change in the 

 properties of aqueous vapour, or in the condition of 

 our atmosphere, would effectually prevent the pos- 

 sibility of a glacial epoch occurring on our earth, 

 notwithstanding a high state of eccentricity, we need 

 not wonder that the planet Mars is not in a state of 

 glaciation. But the eccentricity of Mars, though high, 

 is still far from its superior limit, and the planet may 

 yet, for any thing which we know to the contrary, 

 pass through a glacial epoch. 



3rd. Another prevailing misapprehension is the 

 supposition that the theory does not recognize the 

 necessity for geographical conditions. In reading 

 " Island Life " one might be apt to suppose that one of 

 the chief points of difference between Mr. Wallace and 

 myself is that he regards geographical distribution of 

 sea and land as an important factor in a theory of 

 geological climate, whereas I entirely ignore this 



* See ' Climate and Time,' p. 79. 



