HOW LIFE CAME TO EARTH 173 



Helmholtz, and definitely expresses the opinion 

 that, " Dead matter cannot become living 

 without coming under the influence of matter 

 previously living. This seems to me as sure 

 a teaching of science as the law of gravitation." 



On the other hand, the great botanist, 

 Nageli, taught, " If in the physical world all 

 things stand in causal connection with one 

 another, if all phenomena proceed along 

 natural paths, then organisms, which build 

 themselves up from and finally disintegrate 

 into the substances of which inorganic nature 

 consists, must have originated primitively 

 from inorganic compounds. To deny spon- 

 taneous generation is to proclaim a miracle." 



Not only do the adherents to the several 

 variants of this story of the advent of life 

 upon the earth from space, differ as to whether 

 life has lasted from all time, or was subse- 

 quently created, there are many variations 

 as to how it came. Some, such as Kelvin 

 and Helmholtz, regard it as carried by meteor- 

 ites, or fragments of planets that had borne 

 life when they went to destruction ; others, 

 such as Richter, or more recently Arrhenius, 

 postulate an impalpable dust or panspermia 

 scattered through all space and borne from 

 the atmosphere of one planet to that of 

 another. Kelvin may be taken as the ex- 



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