The Experimental Method 5 



conditions. In fact the control of natural phenomena is the 

 goal of experimental work. 



In the studies of physics and chemistry the method of experi- 

 ment is so familiar, that we think of their advancement as 

 taking place by experiment alone. In biology the situation is 

 different, and new discoveries are looked for as often in the 

 field of observation as of experiment. This difference is due to 

 the higher stage of development that has been reached by the 

 physical sciences, while biology is still, in large part, in the 

 lower stages of its evolution where facts are insufficiently known. 

 Nevertheless the amount of time still given to descriptive work 

 is out of proportion to the present condition of development of 

 biology. 



A few examples may serve to illustrate the differences be- 

 tween the descriptive and the experimental study of zoology. 

 The egg of an animal, if set free and fertilized, begins at once 

 to develop. Descriptive embryology gives us the different stages 

 through which the egg passes, but no matter how complete the 

 description, we still know little or nothing of the causes that 

 are operating to bring about the development. What, for 

 instance, does the spermatozoon bring into the egg to make it 

 develop? What physical and chemical changes take place 

 during cleavage ? What makes the embryo turn in at one pole ? 

 Why do certain cells develop cilia ? These and a hundred other 

 questions suggest themselves. Observation has failed to answer 

 them. 



Another method employed in recent years has been to attempt 

 to find certain physical or chemical changes that seem to be 

 similar to those observed in the developing egg. Thus it has 

 been suggested that the spermatozoon brings a ferment into the 

 egg ; that the cleavage is due to differences of surface tension ; 

 and that the gastrulation is caused by osmotic pressure. Ma- 

 chines that behave in somewhat similar ways have even been 

 constructed to illustrate some of these changes. Interesting as 

 the ideas derived from these sources may be, their scientific 

 value lies only in their suggestiveness, until it can be shown 



