206 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS 



mentation. Hence, the dictum of an earlier time, " omne vivum 

 ex ovo," every living thing from an egg, and the later one, " omnis 

 cellula e cellula," every cell from a cell, express the facts as now 

 established. The manner in which unicellular organisms arise by 

 processes of reproduction from parent organisms like themselves 

 has been described. The origin of higher animals by reproduc- 

 tion and development is explained in subsequent chapters. 



The long controversy over spontaneous generation, described 

 in the foregoing paragraphs, was related throughout to the idea that 

 certain diseases spread and multiplied like living organisms. 

 When it was discovered that organisms did cause disease, this idea 

 was confirmed. It is, therefore, appropriate to consider the 

 Germ Theory of disease in the section that follows. 



Protozoa and Disease 



The Germ Theory of Disease. — What is known as the Germ 

 Theory of disease postulates that certain diseases are produced 

 by " germs," or minute organisms that hve as parasites in the bodies 

 of plants or animals and are the specific causes of particular dis- 

 eases. The " symptoms " of the disease are, in general, the bodily 

 reactions in response to such parasitic invasions. Some diseases, 

 like the hookworm disease, are caused by larger parasites; others, 

 hke malaria and typhoid fever, by minute parasites which are pro- 

 tozoa or bacteria. In general, all " infectious " diseases are 

 due to organisms which infect the body and whose normal exist- 

 ence in this phase of their life cycle is a parasitic one. 



Malaria and the Malaria Parasite. — The disease called malaria 

 has been known since ancient times. Some have even regarded 

 it as one of the causes of the decHne of ancient Rome. Certain it is 

 that both Romans and Greeks, and probably other ancient peoples, 

 suffered greatly from this pestilence. Early explorers of the 

 Americas found it estabhshed in the tropical regions of both con- 

 tinents and brought back to Europe the South American Indian's 

 medicine in the form of " Peruvian bark," from which quinine was 

 later extracted. At the present time the seriousness of the dis- 

 ease in all the warmer parts of the world is a matter for states- 

 manship as well as medicine. In India alone there are over a 

 million deaths per year, to say nothing of the uncounted thousands 

 who are incapacitated. Other regions are not much better off, 



