THE ORIGIN OF LIFE 5 



the earlier steps in the evolution of liv- 

 ing forms from lifeless matter. This 

 prediction has been verified in so far as 

 we now know a considerable number of 

 such forms (filterable viruses) some of 

 which cause important diseases. They 

 can not be detected by the ordinary mi- 

 croscope; they pass through filters which 

 retain all the ordinary bacteria. But we 

 do not think of them as lending support 

 to the doctrine of spontaneous genera- 

 tion, since there is no proof that they 

 can arise from lifeless material. 



How then did life originate.^ Are we 

 not forced to assume that somewhere, 

 at some time, spontaneous generation 

 must have taken place .'^ Although no 

 such process appears to occur at present 

 we may nevertheless suppose that in 

 earlier geological epochs and under more 

 favorable conditions it might have hap- 

 pened. And if, as Arrhenius supposes, 

 life can originate on any appropriate 

 heavenly body and spread thence to 

 other bodies we have an immense extent 



