THE INDIVIDUAL AS A MEMBER OF A RACE 209 



Although Redi's experiments marked the beginning of a growing dis- 

 belief in spontaneous generation, they by no means settled the question. 

 About the time he was making his experiments, another naturalist 

 (Leeuwenhoek) was beginning to make the powerful, simple lenses with 

 which he was to discover the existence of the minute living organisms 

 that we know today as the protozoa and bacteria. The first studies on 

 these minute forms of life strongly suggested that they could arise by 

 spontaneous generation, even though flies, worms, and frogs did not. It 

 was found that one could always obtain such microscopic life simply by 

 placing some clean, dry hay in a glass of clean, clear water. Soon after the 

 water became more or less discolored by soluble substances from the 

 hay, it would be found to be swarming with bacteria and later with 

 protozoa, and even if the infusion were boiled, living organisms would 

 appear a few days after it cooled. Such observations did much to offset 

 the results of Redi's experiments on flies, for they at least seemed to 

 support in principle the possibility of spontaneous generation. 



Gradually, however, as compound microscopes became practical in- 

 struments and provided detailed images at high magnification, and as 

 appropriate techniques were devised for the study of microscopic life, 

 observations to test the spontaneous generation of even protozoa and 

 bacteria became possible. About 1780, Spallanzani was able to isolate a 

 single bacterium and watch it divide to form first two and then four 

 "daughter" bacteria. He also made numerous experiments on sterilized 

 (boiled) infusions that convinced him that even protozoa and bacteria 

 could not arise spontaneously. Such experiments, however, call for a 

 much more elaborate and precise technique than those that served Redi 

 in his experiments with flies, and it was not until after the work of Pasteur, 

 Cohn, Koch, and Tyndall, 1 in the latter half of the nineteenth century, 

 that all biologists were convinced that spontaneous generation was defi- 

 nitely disproved for all forms of living organisms. 



Today it is fully established that every living organism comes into 

 existence through the reproductive functioning of parent organisms (or a 

 single parent organism) quite like itself. It follows that each and every 

 kind of organism that exists on the earth today is but a recent link in an 

 unbroken sequence of once living, reproducing individuals. We do not 

 know how life originally came into existence; but whether it was pro- 

 duced by a special creation by some supernatural power, or through 

 spontaneous generation in some earlier and different stage of the earth's 

 history, or whether the first living organisms were somehow transported 

 to the earth from another planet, we are certain that under the conditions 

 that now prevail "all life comes from life." 



1 The application of their work to surgery was first made in the 1860s by Lister, in 

 the introduction of "antiseptic surgery." 



