ON THE ORIGIN OF THE PLANETARY SYSTEM. 197 



up of its mother-body, and had not shared the general rise 

 of temperature, it must necessarily have first passed through 

 the atmosphere of the earth, before it could deliver itself of 

 its organisms for the purpose of peopling the earth." 



* Now, in the first place, we know from repeated observa- 

 tions that the larger meteoric stones only become heated in 

 their outside layer during their fall through the atmosphere, 

 while the interior is cold, or even very cold. Hence all 

 germs which there might be in the crevices would be safe 

 from combustion in the earth's atmosphere. But even those 

 germs which were collected on the surface when they reached 

 the highest and most attenuated layer of the atmosphere would 

 long before have been blown away by the powerful draught 

 of air, before the stone reached the denser parts of the gaseous 

 mass, where the compression would be sufficient to produce 

 an appreciable heat. And, on the other hand, as far as the 

 impact of two bodies is concerned, as Thomson assumes, 

 the first consequences would be powerful mechanical motions, 

 and only in the degree in which this would be destroyed by 

 friction would heat be produced. We do not know whether 

 that would last for hours, for days, or for week^. The frag- 

 ments, which at the first moment were scattered with planet- 

 ary velocity, might escape without any disengagement of 

 heat. I consider it even not improbable, that a stone, or 

 shower of stones, flying through the higher regions of the 

 atmosphere of a celestial body, carries with it a mass of air 

 which contains unburned germs. 



1 As I have already remarked I am not inclined to suggest 

 that all these possibilities are probabilities. They are ques- 

 tions the existence and signification of which we must re- 

 member, in order that if the case arise they may be solved 

 by actual observations or by conclusions therefrom.' 



