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 zodlogy. 
The egg of an animal, if set free and fertilized, begins at oncc 
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 spermatozoén 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 spermatozodn 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 
