602 



COLLEGE ZOOLOGY 



periment of the Italian Redi in 1680 ex- 

 ploded this theory in regard to the larger 

 types of animals. Redi placed meat in wide- 

 mouthed flasks; some flasks were left open, 

 some were covered with gauze, and others 

 with paper. The meat decayed in all vessels. 

 Flies entered the open vessels and laid eggs 

 which hatched into maggots. No larvae de- 



veloped in the meat of the other vessels; 

 but on the cloth of those covered with 

 gauze, flies laid eggs which developed into 

 maggots. Thus it was concluded from this 

 experiment that the maggots arose from the 

 eggs of flies and not from decaying meat 

 (Fig. 426). 

 When microscopes were developed so 



Uncovered 



Covered v/ith 

 gauze 



Covered with 

 paper 



Pasteur's flask 



Figure 426. Spontaneous generation. Diagrams illustrating methods of disproving this theory. 

 Left, the 3 flasks illustrate Redi's method of experimenting with blow flies. Right, Pasteur's flask 

 as used in his experiments. This peculiar glass flask was completely closed except for an opening 

 in the long glass tube on the right side. The flask was half filled with water, containing large 

 numbers of microscopic organisms. By boiling at intermittent intervals, all the living organisms 

 were killed, and the entire contents of the flask made sterile. At the same time, escaping steam 

 condensed, and water filled the lowest part of the bent tube, so that a complete barrier was 

 formed there. The water in the bent tube captured all organisms so that air from the outside 

 which finally reached the sterile mixture at the bottom of the flask was sterile. The result was 

 that within the flask proper no life developed, whereas in the water barrier in the bent tube a 

 host of minute organisms developed. 



that ver}' minute organisms such as proto- 

 zoans and bacteria were made visible, the 

 advocates of spontaneous generation claimed 

 that these tiny creatures arose from nonliv- 

 ing matter. Pasteur (1864) finally disproved 

 their contention by protecting sterile hay in- 

 fusions from the microorganisms in the air 

 (Fig. 426); such infusions became popu- 

 lated if exposed to the air, but remained 

 sterile if the air was heated or all particles 

 were removed from the air before it reached 

 the infusion. 



Biogenesis 



The theory of spontaneous generation, 

 abiogenesis, thus gave way to that of bio- 



genesis, which maintains that all life arises 

 from preexisting life. If living animals do not 

 arise from nonliving matter, it is natural to 

 inquire how the world became populated 

 with them, since both geologists and astron- 

 omers tell us that at one time, long ago, 

 life could not have existed on the earth. The 

 doctrine of special creation, that is, that 

 each species of animal was specially created, 

 is sufficiently refuted to the satisfaction of 

 most biologists by the facts of organic evolu- 

 tion. Life must, therefore, have originated 

 on the earth from nonliving matter, or it 

 must have been brought to the earth from 

 some other part of our universe. The latter 

 idea, known as the cosmozoic theory, is so 

 improbable as to be hardly worthy of con- 



