172 ORIGIN AND NATURE OF LIFE 



with slight variants in each case, is that 

 known as the theory of cosmozoa or pan- 

 spermia. According to all the many versions, 

 life did not spontaneously arise on the earth, 

 but was carried to it from some other world 

 where it already was existent, either by 

 origin there, or by similar carriage. The 

 various forms of this belief (for it cannot be 

 called more in lack of all experimental evi- 

 dence) differ as to whether life was coeval 

 with matter and had existed for ever, or 

 whether it was once created on a planet 

 and ever afterwards disseminated. Helmholtz 

 puts the problem clearly when he says, " I 

 cannot contend against one who would 

 regard this hypothesis as highly or wholly 

 improbable. But it appears to me to be 

 a wholly correct scientific procedure, when 

 ail our endeavours to produce organisms out 

 of lifeless substance are thwarted, to question 

 whether, after all, life has ever arisen, whether 

 it may not be even as old as matter, and 

 whether its germs, passed from one world to 

 another, may not have developed where they 

 found favourable soil. The true alternative 

 is evident ; organic life has either begun to 

 exist at some one time, or has existed from 

 eternity." 



Lord Kelvin held similar views before 



