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 exactly as in normal 

 fertilization, the hypothesis becomes at least probable that a fer- 

 ment is brought in by the spermatozoon 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 interest 



