PHYLOGENY 



THEORIES OF THE ORIGIN OF LIFE 



Spontaneous Generation. Of the many theories of the origin of hfe, 

 perhaps the oldest is the theory of spontaneous generation, according to 

 which even comphcated forms of Hfe might arise spontaneously from non- 

 living matter. Thus Aristotle believed that mosquitos and fleas arose from 

 putrefying matter. Tadpoles, worms, and many other small organisms 

 have been supposed to arise from mud. Flies were supposed to be formed 

 from putrefying flesh. Before the motor age, every child heard that a 

 horsehair, left in water, would transform into a horsehair worm. And meal 

 worms have often been supposed to arise from flour spontaneously. Even 

 such large and complex animals as rats have been supposed to arise spon- 

 taneously from nonliving matter. Redi, an Italian physician of the seven- 

 teenth century, attacked the theory of spontaneous generation experimen- 

 tally, and he left it badly damaged. He exposed meat in containers which 

 were covered over with fine mesh cloth. No maggots appeared on the 

 putrefying meat, but flies laid their eggs on the cloth covers, and maggots 

 developed there. It was evident, then, that the maggots which ordinarily 

 appeared in spoiling meat were not spontaneously produced, but were 

 developed from eggs laid in the meat by adult flies. A century later, simi- 

 lar experiments were performed by the Italian priest Spallanzani, who 

 also showed that, if meat were boiled in a sealed container, no organisms 

 developed in it, even if it had been previously infected. This fact was soon 

 applied to the practical problem of food preservation by means of can- 

 ning. 



After the work of Redi and Spallanzani, the theory of spontaneous 

 generation no longer commanded the respect of biologists. But the dis- 

 covery of bacteria changed this. Here were organisms simpler than any 

 previously imagined. Their occurrence was practically universal, and it 

 was very difficult to exclude them from any medium suitable for their 

 growth. The possibility that they might be produced spontaneously witliin 

 every sort of organic medium was most suggestive, and it had many ad- 

 herents. The famous experiments of Pasteur disproved this completely. 

 Boiled broth was kept in a closed container, with air entering by a capil- 

 lary tube which was bent to form a trap for solid particles. Thus the broth 

 was freely exposed to oxidation, yet no bacteria appeared in it. Hence it 

 was evident that air-borne bacteria ordinarily infected exposed broth (or 

 other suitable media), and that the bacteria themselves arose only from 

 pre-existing bacteria. This dealt the death blow to the theory of spon- 

 taneous gencMation of complex organisms. 



The Cosmozoic Theory. A second theory of the origin of life is the 

 cosmozoic theory, that the original spores of life reached the earth acci- 

 dentally from some other part of the universe. This theory is completely 

 unsatisfactory for two reasons. First, because of the intense cold, extreme 

 dryness, and the intense radiation of interstellar space, the probability 

 that even the most resistant of living spores could withstand exposure to 

 interstellar space is vanishingly small. Second, the theory does not explain 



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