6 Experimental Zoology 
that the changes in the embryo are really the outcome of similar 
processes; and the only way in which certainty can be gained 
on this point is by experiment on the organisms themselves. 
If a ferment starts the development it is our duty to isolate it, 
and to introduce it hypodermically into the egg. If this could 
be done and the development thereby started exacily as in normal 
fertilization, the hypothesis becomes at least probable that a fer- 
ment is brought in by the spermatozo6n and starts development. 
Similarly, for cleavage, for gastrulation, and for every stage 
in the development, experiment alone can give a satisfactory 
answer. 
The essence of the experimental method consists in requiring 
that every suggestion (or hypothesis) be put to the test of experi- 
ment before it is admitted to a scientific status. From this point 
of view the value of an hypothesis is to be judged, not by its 
plausibility, but by whether it meets the test of experiment. Its 
use is therefore primarily for the investigator, and not for the 
layman; yet as a matter of fact the wildest speculations are 
likely to be the ones that excite most popular attention and 
applause. 
It is sometimes said that an hypothesis is useful in proportion 
to the number of facts it brings under one point of view. This 
is true for the student rather than for the investigator. Such 
an hypothesis may have no scientific status. It belongs rather 
to a system of mnemonics. To the teacher, also, hypotheses are 
useful in arousing the interest of his hearers, so that by exciting 
their undeveloped imagination he can make his dry facts more 
entertaining. But let us be careful to distinguish between the 
forensic and the scientific value of hypotheses. 
Hypotheses may be useful, and have been used in various 
ways. They have been used, as just stated, to hold together a 
body of isolated facts; as such they are in reality only fictions. 
They have been used in the reconstruction of supposed historical 
events, especially in biology in the setting up of family trees. 
In this case they can only claim to be more or less plausible 
suggestions. Hypotheses have been used to direct intercst 
to 4 4 4 
