4 LIFE: ITS NATURE AND ORIGIN 



living unit, the probability of its appearance is vastly increased by 

 three factors: 



(1) the enormous numbers of "card packs" (i.e., mixtures of 

 atoms and molecules) experimented with over the whole 

 earth; 



(2) the equally large numbers of "shufflings" of these molecular 

 mixtures over geological epochs, and at high rates of speed; 



(3) the inherent structure of material units, which rules out 

 many, if not most, of the theoretically calculable possibilities. 



But the fact remains that living units did arise and undergo 

 evolution in a most marvelous manner, even though scientists, 

 gradually emerging from ignorance, are unable in their few 

 human generations to understand all the biological details of the 

 past, much less to duplicate them experimentally. We have not 

 yet discovered a synthetic medium in which to grow viruses or 

 even the leprosy bacillus, although the latter is closely related to 

 the tubercle bacillus which grows well in Long's synthetic 

 medium. 



The probability that any living unit will originate spontane- 

 ously increases enormously with its simplicity. Since we know 

 neither the nature of the hypothetical first living unit, the nature 

 of the then-existing milieu, nor the value of the three factors 

 mentioned above, we have slight basis for calculating the proba- 

 bility for the chance formation of such a unit. We cannot, even 

 in imagination, construct a complete picture of terrestrial con- 

 ditions when life first appeared, say a billion or more years ago. 

 The composition of the earth's atmosphere; solar radiation and 

 its ability to reach the earth's surface; the alpha, beta, and gamma 

 radiations from radioactive atoms — all must have been quite dif- 

 ferent from what they are at present. Other factors may be 

 unknown or unweighted; for instance, fourteen hundred million 

 years ago the amount of uranium isotope 235 was about four 

 times what it is now. 



When once formed, a "living molecule" could serve as a mold 

 or template for the production, not only of incalculable numbers 

 of like molecules but also of new types arising from modifications 

 of the older ones. The initial absence of predators must have 

 been a most favorable factor; for though the spontaneous gener- 

 ation of living units may even now be continually occurring at 

 the ultramicroscopic level, existing and well-established forms of 

 life make their chance of persistence very slight, either by devour- 



