ee SELECTION AND PREPARATION OF ORGANISMS 



tion has been collected. This background information may offer an 

 advantage over a less widely studied form. The laboratory rat and mouse, 

 the rabbit, corn (maize), Chlorella, the bacterium Escherichia coli, and 

 several other species are all covered by a tremendous amount of litera- 

 ture. If there is any question about the compatibility of a certain organ- 

 ism with the problem under investigation, a search through this literature 

 may provide the answer. 



Genetic Backgrotmd: The genetic background of the animals or plants 

 may be important. Usually genetic homogeneity is desirable in that it 

 decreases the natural variation among the organisms. Because the rats 

 and mice in common laboratory use have been inbred for many genera- 

 tions, there is relatively little genetic variation. Organisms with little 

 genetic variability are not as strong as "wild types," however, and, in at 

 least one of the strains of laboratory rats, "wild rats" are introduced at 

 planned intervals to maintain vigorous stock. 



If records of genetic background or pedigrees are known, so much 

 the better. One of the largest groups of pedigreed dogs is maintained at 

 the University of Utah for use in studies on the effects of radiation. 

 Since each dog's ancestors are known for several generations, it is pos- 

 sible to relate inheritance with susceptibility to radiation damage. 



Avoidance of Unnecessary Complications: Usually biological experi- 

 ments are difficult enough without the addition of unnecessary compli- 

 cations. If it is possible at all, it is desirable to avoid these complications. 

 Many of the early investigations on cell metabolism used yeast cells be- 

 cause they are simple, complete organisms. One of the advantages of 

 Chlorella in photosynthesis studies is that each cell is a unit in contact 

 with the experimental environment. There is almost no direct interac- 

 tion between cells. In higher plants, and even more in higher animals, 

 the internal organization is so complex and there is such close inter- 

 relationship betweens cells that the interpretation of experimental results 

 is often difficult. 



Representation of General Group: It is desirable that the experimental 

 organism be representative of a general group. Rats are more-or-less 

 typical mammals; sunflowers are typical flowering plants. Exceptional 

 behavior may be interesting, but it may contribute Httle to our under- 

 standing of general biological principles. If we are to induce the gen- 

 eral principles from the experimental results, the ordinary is preferable 

 to the unique. As an example, a great to-do is raised about insectivorous 

 plants although they are relatively rare (except in pictures in biology 

 texts). Experiments have been performed on these plants, but they 

 contribute only to our knowledge of insectivorous plants and very little 



