54 THE AMERICAN NATURALIST. (VoL. XXXIX. 
it is at present, to separate fact from theory, to examine each hypoth- 
esis, discarding the false and emphasizing the probably true. Be- 
sides the enormous amount of reading required, Pfeffer and his 
assistants have done a great deal of experimental work, testing in his 
own laboratory the work and conclusions reported by others. The 
result is a critical mastery, on Pfeffer's part, of the whole subject of 
plant physiology, such as no other man possesses, and the fruit of 
this mastery is his book. The book, in presenting the science as it 
now is, shows what is known and thought, how much more this is 
than when the first edition was published nearly twenty-five years 
ago; it shows also how indefinite our knowledge is, how inadequate 
our thoughts, and how limited the Geld of investigation has hitherto 
been. As remarked in a previous review, the plant physiology of 
to day is the result of the study of higher plants mainly. The lower 
plants have been more carefully studied by physiologists during the 
last five years than ever before. The results of this study are evi- 
dent and valued. When the physiology of these plants is understood 
even to the extent to which we now understand the physiology of 
higher plants, plant physiology will bear a different aspect from what 
it does to-day. In the second as in the first edition, Pfeffer divides 
his subject into Stoffwechsel and Kraftwechsel. To these classical 
headings a third is now sometimes added, Farmwechsel. Though 
_ this last is but a special aspect, a special result, of the other two, it 
is a result which will certainly become increasingly important as it 
becomes increasingly evident from the experimental investigation of 
simpler plants. 
From Pfeffer's book it is clear that the study of the influence of 
the various factors of the environment upon the form of living : 
organisms stands, in results, far behind our knowledge of these fac- 
tors upon the immediate behavior of living organisms. In this last 
part of the Handbuch we have mainly the subject of movements. 
These are examined in detail. The mechanics of movement are 
better understood than the action of the stimulus upon the living 
organism, but in both cases the phenomena are so complicated and 
often so contradictory in different forms that it is impossible to make 
a general statement of the subject which would be both clear and 
truthful. For this reason this part of Pfeffer's book possesses in the 
extreme those qualities for which the earlier parts have been most 
severely criticised, viz., lack of definite summary statements, repeti- 
tions, and minute argumentative criticism. Summary statements 
which would be true cannot now be made. Repetition is unavoid- 
