4 o PERIOD II. 



supposition of a "vivifying" principle," which generated 

 living things of itself a supposition contrary to the 

 truer doctrine which he taught elsewhere. Vallisnieri 

 was able to explain how the egg is introduced into the 

 rose-gall, which a little later shows no mark of injury ; 

 while Malpighi examined the young nut and found both 

 hole and egg. How parasitic worms reach the brain- 

 case of the sheep could be explained only in a later 

 age. Meanwhile Swammerdam, Leeuwenhoek, Reau- 

 mur, and many other special students confirmed and 

 extended Redi's experiments on the blow-fly ; and every 

 fresh instance of normal generation in a minute organ- 

 ism did something to weaken the belief in spontaneous 

 generation. 



Late in the eighteenth century that belief revived in 

 a form less easy of refutation. Leeuwenhoek had 

 discovered that organic matter putrefying in water often 

 yielded abundance of microscopic organisms of the most 

 diverse kinds, many of which could resist drying in 

 air and resume their activity when moistened again. 

 BufFon, ever ready with a speculative explanation, 

 maintained that such minute organisms were spon- 

 taneously generated, and that they were capable of 

 coalescing into bodies of larger size and more complex 

 structure. Needham supported Buffon's theories by 

 experiments. Taking infusions of meat, corking 

 them, and sealing them with mastic, he subjected 

 them to a heat which he thought intense enough to 

 destroy life ; after an interval the microscope revealed 

 an abundance of living things which he affirmed to have 

 been generated from dead matter. Spallanzani repeated 

 Needham's experiments with stricter precautions, sealed 

 his flasks by fusing their necks in a flame, and then im- 

 mersing them in boiling water until they were heated 



