EVIDENCES— EXISTING ORGANISMS 115 



determiners of individual "existence are seen to apply to any phase 

 of individual activity. Even the production of a new individual 

 demands first of all inherent fitness, second, the proper environ- 

 ment, and third, the proper response. Some individuals are con- 

 genitally unable to produce germ cells. In most species repro- 

 duction occurs only under definite conditions. Finally some 

 individuals reproduce in response to certain conditions while others 

 do not. When once produced the individual has a certain range of 

 possibilities. Within the same species these possibilities may vary, 

 but in the main they fall between certain extremes in most indi- 

 viduals. The different responses of various individuals within this 

 range are known as variation. It is so prevalent that no two indi- 

 viduals of any species are exactly alike, and so obvious that it 

 has been called the most invariable thing in nature. 



Most species come into contact with environment common to 

 various other species. Not all species respond to the same condi- 

 tions in the environment, or like individuals, they may respond 

 to them in different ways, depending on their inherent powers. 

 In some cases the possibilities of the species are such that any indi- 

 viduals may accomplish the same end in either of two different 

 ways. So it is with Euglena, which can, if light is lacking but 

 organic food is available, live as an animal, although under normal 

 conditions it is equally able to live as a plant. The inherent possi- 

 bilities of an organism may be such as to enable it under different 

 environmental conditions to live in very different ways. 



Variability of Organisms. That difference of response results 

 in qualitative differences in the organisms is evident in the same 

 organism, Euglena, which loses its chlorophyll when it lives in the 

 dark as an animal. Within our own lives we see similar evidences 

 in the development of calluses through constant friction or 

 pressure, and the increase in size of muscles through use. The 

 converse is equally true, that disuse of a part results in diminution 

 of its powers. These effects of use and disuse were emphasized by 

 Lamarck as a part of his theory of evolution. 



The degrees in which organisms are able to respond to varying 

 conditions are in themselves variable. It has been recorded, for 

 example, that in streams where dipnoid fishes are found, the cessa- 

 tion of flow and stagnation of the pools remaining in the river bed 

 are sometimes fatal to ordinary gill breathing fishes, while the 

 Dipnoids, because of their ability to secure abundant oxygon at the 



