526 TEXTBOOK OF BOTANY 



meats produce no maggots, that rags and meal inside stoppered bottles 

 never give rise to mice, and that frogs develop from frog eggs and not 

 from mud. Although a few individuals as earlv as the 17th centurv defi- 

 nitely expressed the opinion that organisms arise only from preexisting 

 organisms, it was not until after the middle of the 19th century that the 

 theory of spontaneous generation was shown to be utterly false as an 

 explanation of the origin of fullv fomied plants and animals. 



Speculation. Early Greek philosophers, including Aristotle (384-322 

 B.C. ) , all subscribed in their writings to the idea of spontaneous develop- 

 ment of living organisms. Their statements were copied and enlarged 

 upon by nearly all the medieval scholars of Europe. For example, there 

 was then a widespread belief in the "goose tree" and the "vegetable 

 lamb." Geese and ducks were thought to be formed either directly from 

 the fruit of certain trees, or from sea shells borne by these trees. Current 

 also were the so-called observations that certain trees bore melon-like 

 fruits containing fully formed lambs. Such incredible accounts as these 

 were widely accepted until early in the 18th century. 



Early experimentation. During the 16th century a few investigators 

 with a desire to experiment for themselves showed that spontaneous 

 generation of some organisms was clearly erroneous. Van Helmont 

 ( 1577-1644), although he thought his experiment proved that mice came 

 from wheat grains, was troubled because these mice were just like those 

 borne bv a female mouse. 



The Italian phvsician Redi (1626-1697) proved experimentally that 

 "woiTns" in meat are fly larvae. He further showed that if the flies are 

 kept from depositing eggs on meat, no larvae develop in it. Redi did not, 

 however, draw the inference that this experiment disproved spontaneous 

 generation. Indeed, he suspected that "worms" in plant galls arose spon- 

 taneously from plant juices. Vallisneri (1661-1730) soon afterward 

 proved that these "worms" likewise developed from the eggs of insects. 



Leeuwenhoek and his microscopes. The Dutch inventor of the micro- 

 scope, Leeuwenhoek (1632-1723), looking through his simple magni- 

 fiers, saw organisms invisible to the unaided eye. He examined all sorts 

 of infusions containing microorganisms, and left accurate descriptions, 

 and many remarkable figures of bacteria, yeasts, algae, and many other 

 organisms. Leeuwenhoek did not subscribe to the theory of spontaneous 

 generation, but insisted that the tiny organisms he saw grew from sim- 

 ilar organisms that got into his infusions from the air. 



