70 ANIMAL STUDIES 



birth of Christ that these beliefs were attacked and be- 

 gan to be given up. William Harvey, an English natural- 

 ist, declared that every animal comes from an egg, but that 

 the egg might "proceed from parents or arise spontane- 

 ously or out of putrefaction." In the middle of the same 

 century Eedi proved that the maggots in decaying meat 

 which produce the flesh flies develop from eggs laid on the 

 meat by flies of the same kind. Other zoologists of this 

 time were active in investigating the origin of new indi- 

 viduals. And all their discoveries tended to weaken the 

 belief in the theory of spontaneous generation. 



Finally the adherents of this theory were forced to 

 restrict their belief in spontaneous generation to the case 

 of parasites and the animalcules of stagnant water. It was 

 maintained that parasites arose spontaneously from the 

 matter of the living animal in which they lay. Many para- 

 sites have so complicated and extraordinary a life history 

 that it was only after long and careful study that the truth 

 regarding their origin was discovered. No case of spon- 

 taneous generation among parasites is known. If some 

 water in which there are apparently no living organisms, 

 however minute, be allowed to stand for a few days, it will 

 come to be swarming with microscopic plants and animals. 

 Any organic liquid, exposed for a short time, becomes 

 foul through the presence of innumerable bacteria, etc. 

 But it has been certainly proved that these organisms are 

 not spontaneously produced by the water or organic liquid. 

 A few of them enter the water from the air, in which there 

 are always greater or less numbers of spores of microscopic 

 organisms. These spores germinate quickly and the rapid 

 succession of generations soon gives rise to the hosts of 

 bacteria and Protozoa which infest all standing water. 

 If all the active organisms and inactive spores in a glass 

 of water are killed by boiling the water, " sterilizing " it$ 

 as it is called, and this sterilized water be 'put into a 

 sterilized glass, and this glass be so well closed that germs 



