6 SCIENCE 



of observation, which might be called descriptive biology, simply studies 

 and watches living things as they occur naturally and then describes 

 these organisms. The pattern of distribution of a group of animals over 

 a part of the earth's surface would be studied this way. The biologist, 

 noting and recording most of the climatic, geological, and biological 

 features of this area, would then be in a position to explain what factors 

 influence the distribution of these animals. Or, as another example, the 

 careful study, measurement, and description of a large number of indi- 

 vidual plants could lead a botantist to an understanding of the natural 

 kinship of various groups. Indeed, several hundred years of observations 

 of this type have produced much of what is known as biology. 



The zoologist who studies the distribution of animals by direct ob- 

 servation might ask himself, "What would happen to the mice if erosion 

 removed the topsoil from this mountainside?" He can answer his ques- 

 tion only by waiting for this disaster to occur, or, sometimes by finding 

 a similar mountain where it has already occurred. His observations are 

 limited; he is helplessly dependent on the whims of nature. 



In certain phases of science the experiment offers a means of avoid- 

 ing this difficulty. An experiment is merely an observation in a con- 

 trived, artificial situation. Imagine how much more efficient the process 

 of observation becomes if the scientist asks himself, "What would hap- 

 pen if . . . ?" and then makes the "if" happen. What would happen 

 if a fly with red eyes were to mate with a fly with white eyes? What 

 would happen if a plant were to grow on a soil deficient in nitrogen? 

 What would happen if a part were separated from a living cell? Would 

 it continue its activity? The answers to these questions can be found 

 quickly and efficiently by experiment. The observer has some control 

 over his observations. He can prepare a set of circumstances of his own 

 choosing and then observe the results at his convenience. 



Some aspects of biology are not easily observed by experiment, but, 

 more and more, modern biology is using this useful observational tool 

 which was borrowed from the physical sciences. Most of our recently 

 acquired knowledge about the activities of living cells, most of what we 

 know of genetics and inheritance, and most of what we have learned 

 about the organization and coordination in plants and animals was 

 learned by experiment. 



The ideal experiment requires imagination and careful planning on 

 the part of the investigator. The results of the experiment must be 

 explained, of course, and the "controlled experiment" is an application 

 of logic which makes the explanation easier. Usually the "control" is 

 another experiment, performed at the same time, in which most but 



