The Experimental Method 7 
toward certain fields of study. As such they have often proven 
stimulating and have been useful in acting as a guide for others. 
But the hypothesis of real importance is the working hypothesis 
of the investigator. It is the test by means of which he tries 
to interpret his problem, and therefore it is essential that his 
hypothesis is one having a practical bearing, z.e. an hypothe- 
sis that can be shown to be true or false. It differs in this 
essential respect from purely fictitious and from metaphysical 
hypotheses. 
The working hypothesis carries along with it its dangers as 
well as its advantages; since, while it may lead to discoveries, 
it may, if it is wrong in principle, blind us to the real condi- 
tions. Therefore the investigator must not only be an inventor 
of working hypotheses, but cultivate also a skeptical state of 
mind toward all hypotheses — especially his own —and_ be 
ready to abandon them the moment the evidence points the other 
way. And herein lies one of the differences between the re- 
corder of observations and the experimenter. The work of the 
observer, if exact, is complete in itself, and stands forever as a 
monument to his ability, or at least to his industry; while the 
conclusions of the experimenter, if they are to bear fruit, must. 
become modified with each new discovery. His results are ab- 
sorbed in the current of the next advance, but his consolation 
will be that he has had at least a share in the causal study of 
living things, and in helping the human race toward the control 
of organic phenomena. 
To return to our examples. The growth of animals and 
plants offers a wide field for-experimental study. Under cer- 
tain conditions we see a young animal continuing to grow larger 
until a certain size is reached, when growth slowly ceases. Al- 
though the animal may live for many years longer, it has ceased 
to grow. What makes it grow? Why does it stop growing? 
We have hardly begun experimental work along these lines; 
yet we shall see later that there is a promising field for work in 
this direction. After a time old age comes on and the animal 
dies. We say it dies a natural death, and this seems inevitable, 
