106 THE PLANT 
growth, but they occur to some degree in structure also. It is imperative 
that they be recognized in nature as well as in field and control experiment, 
since they directly affect the ratio between response and stimulus. The 
data which bear upon the similarity of response to extremes of different 
factors are too meager to permit the formulation of a rule. It is permissible, 
however, to suggest the general principle that extreme stimuli produce 
similar growth responses, and to emphasize the need of testing its appli- 
cation to adaptation proper. 
151. The method of working hypotheses. In the study of stimulus 
and response, where the unimpeachable facts are relatively few, and their 
present correlation slight, the working hypothesis is an indispensable aid. 
“The true course of inductive procedure . . . consists in anticipating 
nature, in the sense of forming hypotheses as to the laws which are prob- 
ably in operation, and then observing whether the combinations of 
phenomena are such as would follow from the laws supposed. The investi- 
gator begins with facts and ends with them. He uses such facts as are in 
the first place known to him in suggesting probable hypotheses; deducing 
other facts which would happen if a particular hypothesis is true, he 
proceeds to test the truth of his notion by fresh observations or experi- 
ments. If any result prove different from what he expects, it leads him 
either to abandon or to modify his hypothesis; but every new fact may give 
some new suggestion as to the laws in action. Even'if the result in any 
case agrees with his anticipations, he does not regard it as finally confirma- 
tory of his theory, but proceeds to test the truth of the theory by new de- 
ductions and new trials.” In the treatment of adjustment and adaptation 
which follows, the method of multiple working hypotheses is uniformly em- 
ployed. No apology is felt to be necessary for this, since the whole endeavor 
is to indicate the proper points of attack, and not to distinguish between 
that which is conjectural and that which is known. If an hypothesis occa- 
sionally seem to be stated too strongly, it is merely that it appears, after a 
survey of the problem from all sides, to explain the facts most satisfactorily. 
The final proof of any hypothesis, however, rests not only upon its ability 
to explain all the facts, but also upon the inability of other hypotheses to 
meet the same test. The discovery and examination of all possible hypoth- 
eses, and the elimination of those that prove inadequate are the essential 
steps in the method of working hypotheses. 
1Jevons, W. A. The Principles of Science, 2:137. 1874, 
