430 ELEMENTS OF BIOLOGY 



ous that to describe any of them would be to omit others that are 

 of equal or perhaps greater importance. In general, experiments are 

 designed to test one single condition or effect, as Mendel studied 

 the behavior of a single pair of allelomorphic genes. This requires 

 that all other conditions be kept constant, or that the experiment 

 be checked and controlled by a normal organism maintained under 

 identical conditions except for the treatment required for the experi- 

 ment. The experimental agent or condition is then varied in a 

 known direction and to a known degree. For example, the genet- 

 icist mates plants or animals whose genetic make-ups are known 

 but differ in one or more characters; the physiologist maintains 

 all the conditions of his animal or plant at a constant level and 

 then varies one condition, for example a gland is removed or the 

 temperature of the environment changed. Thus the effects of a 

 single factor or group of factors are isolated and studied by com- 

 parison with the control organisms. The effects are further com- 

 pared with the behavior of non-living systems wherever this is 

 possible. From the results certain hypotheses are advanced con- 

 cerning the nature of the reaction in the organism. These hypotheses 

 are then tested again and again by other types of experiments. If 

 the hypothesis, or its subsequent modifications, has sufficient basis 

 in fact that the results of experiments may be predicted with a high 

 degree of accuracy, it is then known as a principle or law. A law 

 merely expresses the probable basis for regularities that are observed 

 in the life of the organism. From such efforts facts have been dis- 

 covered that have revealed important principles of vital processes; 

 the future will no doubt reveal much more. For human curiosity is 

 insatiable and in laboratories of all civilized countries is a great army 

 of students constantly testing, observing, and training their imagina- 

 tions to project into the unknown. 



Trends of Biology. Biology, like all Science, penetrates the 

 unknown by a succession of advances on different fronts. A par- 

 ticular phase or problem may remain without marked progress 



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