MULTIPLICATION AND DEVELOPMENT 8i 



some naturalists until very recent times. And it was not 

 so easy to disprove the assertions of such believers. 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 swarm with microscopic plants and animals. Any 

 organic liquid, as a broth or a vegetable infusion, exposed to 

 the air for a short time becomes foul through the presence 

 of innumerable microscopic organisms. But it has been 

 certainly proved that these organisms are not spontaneously 

 produced in the water or organic fluid. 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 when they fall into water or some 

 organic hquid, and the rapid succession of generations soon 

 gives rise to the hosts of bacteria and one-celled animals 

 which infest all standing water. If all the active organisms 

 and inactive spores in a glass of water are killed by boiling 

 the water, and this sterilized water be put into a sterilized 

 glass, and this glass be so well closed that germs or spores 

 cannot pass from the air without into the sterilized liquid, 

 no living animals will ever appear in it. We know of no 

 instance of the spontaneous generation of animals, and all 

 the animals whose life-history we know are produced by 

 other animals of the same kind. 



Simplest multiplication and development. — The sim- 

 plest method of multiplication and the simplest kind of 

 development shown among animals are exhibited by such 

 simple animals as Amcsba and Paramoecium. This method 

 we have already studied. The production of new in- 

 dividuals is accomplished by a simple division or fission of 

 the body (a single cell) into two practically equivalent parts. 

 The only change necessary for the young or new Amoeba to 

 become like its parent, is that of simple growth to a size 

 about twice its present size. The development here is 

 reduced to a minimum. Just as the simplest animals per- 



