216 OUTLINES OF EVOLUTIONARY BIOLOGY 



they are themselves first properly sterilized. This sterilization, 

 however, cannot always be effected by a single boiling, for the 

 germs of some organisms are extremely resistant to heat and 

 special precautions have to be taken accordingly. 



Every story of alleged spontaneous generation has, so far, 

 failed to stand the test of scientific criticism and experiment, and 

 accordingly biologists are now almost unanimous in maintaining 

 the contrary doctrine of biogenesis, which teaches that, at the 

 present day at any rate, living organisms arise only by repro- 

 duction from pre-existing living organisms. Nevertheless there 

 must, as we have seen, have been a time when there were no 

 living things on the earth, and therefore living things must have 

 either reached the earth from some outside source or have arisen 

 on the earth itself from matter which was previously not-living, 

 or in other words, by spontaneous generation. It seems as if we 

 had got to take our choice between these two alternatives, neither 

 of which is at all easy to accept. 



It has been maintained by some that organic life is eternal and 

 is transferred from one world to another possibly on meteorites 

 in the form of minute germs or " Cosmozoa," but no one has ever 

 seen such Cosmozoa, and it is difficult to imagine any which 

 would be capable of surviving such a journey. It is perhaps less 

 difficult to believe that under certain conditions, of which we at 

 present know nothing and which perhaps no longer exist on the 

 earth's surface, living protoplasm may have arisen from in- 

 organic matter as the result of chemical and physical processes. 

 In this connection we must remember that protoplasm the 

 physical basis of life contains no chemical elements that are 

 not also found in inorganic matter and is, as a matter of fact, 

 constantly being built up from inorganic constituents in the 

 bodies of living organisms, though apparently only by the action 

 of pre-existing protoplasm. 



Whether physical and chemical forces, such as we are 

 acquainted with, alone sufficed to bring about the transformation 

 of not-living, inorganic material into living protoplasm, or 

 whether, as some people suppose, some unknown " vital force " 

 was (and still is) involved in the process, must remain, for the 

 present at any rate, an open question. 



The time has evidently not yet arrived for solving the great 

 problem of the origin of life, but we may, at any rate, legitimately 

 speculate upon the nature of the living organisms which first 



