i42 



General Biology 



One writer has summed up the external factors that produce or 

 determine reactions as follows : 



"1. The organisms may react to a change even though neither 

 beneficial nor injurious. 



"2. Anything that tends to interfere .with the normal current of 

 life activities produces reactions of a certain sort (negative). 



"3. Any change that tends to restore or save the normal life 

 processes may produce reactions of a different sort (positive). 



"4. Changes that in themselves neither interfere with nor assist the 

 normal stream of life processes may produce negative or positive re- 

 actions, according as they are usually followed by changes that are 

 injurious or beneficial. 



"5. Whether a given change shall produce a reaction or not often 

 depends upon the completeness or incompleteness of the performance 

 of the metabolic processes of the organism under the existing conditions. 

 This makes trie behavior fundamentally regulatory." 



When one organism causes disease in another, it is said to be 

 pathogenic to the organism affected. For example, Amoebae bucallis 

 are found in pyorrhea, a disease of the teeth. The drug emetine kills 

 Amoebae bucallis, and when these are killed, the diseased condition 

 improves. From these facts it has been concluded that this particular 

 protozoan is the cause of pyorrhea, although this is not strictly true. 



While, as we shall shortly see, most of the pathogenic organisms 

 belong to the plant kingdom, still the following animal organisms which 

 cause disease in man, are rather important factors in the study of 

 Biology: 



Fig. 54. 



Entamoeba histolytica from a case of amoebic dysentery in man. Ectp., ecto- 

 plasm; Endp., endoplasm; V, vacuoles; N, nucleus, cy, encysted amoebae. (After 

 Rivas.) 



