Later it was found (I) that there must be several kinds or strains of bac- 

 teriophage, each one specific for a particular species of bacteria, and (2) that 

 bacteriophage will not attack dead bacteria. It is not yet certain whether 

 bacteriophage could increase apart from living bacteria which it eventually 

 destroys, just as living organisms can grow in an artificial broth. From chemi- 

 cal studies, however, it appears that a bacteriophage resembles a virus; that 

 is, it is a "substance" rather than an "organism", although it behaves in some 

 respects like a "hving" something. 



Many chemical compounds that have been produced synthetically re- 

 semble in their behavior complex protein molecules in living things. We can- 

 not call. these substances living. But we can at least imagine that under certain 

 conditions combinations of such unstable molecules bring about a new system, 

 which interacts with other substances as does a virus or a bacteriophage. That 

 is, each makes more like itself out of substances that are different; it assimilates. 

 But we are still far from creating life in a test tube. Indeed, the more we find 

 out about these complex molecules, the less hopeful we are of duplicating any 

 of nature's Hving beings artificially. 



Like the theory that life comes from another planet or another solar sys- 

 tem, the theory of spontaneous generation is concerned with the origin of 

 life in general. It has nothing to say about the beginnings of particular plants 

 and animals. It assumes that whatever makes it possible for living matter to 

 arise from nonliving matter makes it possible also for new forms to develop 

 further with changing conditions. The theory of spontaneous generation thus 

 has a variety of meanings. It depends upon the way we formulate our ques- 

 tion and upon what we assume about the nature of the world or about what 

 makes things happen. 



Our Limited Knowledge If a plant or an animal should some day arise 

 "spontaneously" out of "nonliving" material, we should be quite unable to 

 know about it. Even if a "worm" should crawl out of a lump of mud under 

 our very eyes, we could not tell whether it had developed from an egg or from 

 a grain of sand. All we can say is that, under strictly controlled experimental 

 conditions, nobody has yet seen any evidence of "spontaneous generation". 

 That is, we cannot "prove"''' that spontaneous generation is impossible. We can 

 say only that we have experienced no clear case of it. We are therefore unable 

 to say in advance what may or may not appear from further studies and 

 experiments. 



Did Various Plants and Animals Arise at the Same Time? 



Special Creation What happened between the early period when there 

 was no life on the earth and the later period in which there was life? Some- 

 thing extraordinary must have happened, that is, something that is not fa- 



446 



