264 the president's address. 



to conceive of the solid crust of the earth as the result of a 

 constant interchange of matter between the living and the 

 dead, accompanied by physical and chemical processes of endless 

 complexity. We might even think of it as a huge composite 

 organism, alive only at the surface, but built up on the waste 

 products of its own collective metabolism, like a world-embracing 

 coral reef. I fear, however, that such a conception would be 

 more picturesque than accurate. 



Even if we accepted such a hypothesis we should, of course, 

 have to remember that such a state of affairs could only have 

 arisen through a slow and gradual process of evolution. 

 Whether this process occupied a hundred million or a thousand 

 million years would be a matter of comparatively small import- 

 ance. It would be enough for our present purposes to recognise 

 that it must have had a beginning at some extremely remote 

 period of geological time, when the crust of the earth could not 

 by any possibility have been composed of the detritus of living 

 things. 



It is generally admitted that there are only two possibilities 

 with regard to the origin of terrestrial organisms. Either thev 

 must have been imported from some other planet in the form 

 of germs, or they must have developed on the earth's surface 

 from inorganic materials that formed part of the earth itself. 

 Either event could only have taken place after the earth had 

 cooled sufficiently to permit of the existence of those peculiarly 

 unstable colloidal compounds of which living bodies are composed. 



The first hypothesis has, as you are aware, received the sup- 

 port of no less eminent a man of science than the late Lord Kelvin, 

 who believed it possible that the germs of living organisms 

 might have been brought to the earth by meteorites. The chief 

 objection to this view appears to be the difficulty of believing 

 that any organism could withstand the heat generated by the 

 friction of the meteorite with the earth's atmosphere. 



A modification of the same hypothesis, sometimes known as 

 the Theory of Panspermia, is maintained by Svante Arrhenius 

 and others. According to this theory, numerous living germs of 

 extremely minute size occur scattered through space, derived 

 from various planets upon which life is supposed to exist, 

 though at present we have no proof whatever that life does 

 exist upon any pi met except the earth itself. The nature of 



