202 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS 



Biogenesis vs. Abiogenesis 



HistoricaL — As a result of investigations extending over a 

 period of more than two hundred years, it was shown during the 

 third quarter of the nineteenth century that spontaneous origin of 

 protoplasm does not take place. Protoplasm seems to come from 

 preexisting protoplasm, by a process of detachment, as in repro- 

 duction, or by an increase of bulk as in growth. It was natural 

 for the ancients to believe that animals like rats and mice, 

 frogs, and insects, which suddenly swarmed in certain places, 

 were produced from the mud of the fields under the influence of 

 the sun's rays, or bred spontaneously within the decomposing 

 carcasses in which they were found. It was even supposed that 

 forms Hke the mammals, which developed within the female 

 parent, arose spontaneously under the influence of the male's 

 seminal fluid. The higher animals were, of course, known to have 

 " parents," but there was no concept of the continuity between 

 generations, save as " eggs " like those of birds and reptiles were 

 observed to produce young, and mammals to give birth to living 

 offspring. However, it was gradually established that smaller 

 animals arose from eggs. The ItaHan naturalist, Redi, performed 

 experiments (1688) that showed how maggots originated in meat 

 from the eggs laid by flies. He placed pieces of meat in jars, 

 covering some with wire gauze and others with parchment, and 

 leaving others uncovered. Flies were attracted as the contents 

 of the jars began to decompose, and laid their eggs directly on the 

 meat or upon the wire gauze. Maggots were seen to hatch from 

 these eggs and to develop as they consumed the meat. Eggs 

 transferred from the gauze to the meat behaved in hke manner. 

 The meat in the parchment-covered jars merely decomposed withr 

 out the appearance of any maggots. Redi made other observa- 

 tions upon the development of insects and reached the conclusion 

 that all cases of " spontaneous " generation of Hving organisms 

 were presumably due to the introduction of living germs from 

 without. Had it not been for the discovery of protozoa, bacteria, 

 and other micro-organisms during the latter half of the seventeenth 

 century, the question would perhaps never have arisen again in the 

 subsequent history of biological science. 



In 1676, the Hollander, Anthony van Leeuwenhoek, discovered 

 with the microscope, which had recently come into use as a toy and 



