520 THE AMERICAN NATURALIST. ` [VOL. XXXIII. 
Persons unfamiliar with the present trend of biological research 
will be surprised at the large amount of quantitative work exhibited 
in this record of experiments. Wherever possible, the author has 
pointed out the adaptive character of the reactions observed in the 
laboratory. Some readers may be disappointed to find that he has 
not gone farther than this, and discussed the effects of chemical and 
physical agents upon the forms of animals and plants in the state of 
nature. The reason that he has not done this is, doubtless, in the 
first place, the fact that in nature the effects of these agents upon 
growth is complicated with their effects upon differentiation, which 
will be treated of in a later volume; and, in the second place, the lack 
of material. When the effects of agents under control in the simpli- 
fied conditions of the laboratory are understood, we may hope to 
solve the problem of form as presented under the complex conditions 
of the natural habitat. The work under review is an important step 
in that direction. 
In most cases where the direction of growth is changed in response 
to stimulation, true tropism, the result is to place the organ in a more 
favorable position. It is clearly adaptive. The reactions classed 
under the head of electrotropism, on the other hand, cannot be shown 
to be adaptive, because organisms never meet with the stimulus 
employed in the state of nature. Yet the reaction takes place with 
as much precision as the response to light or heat. 
The most interesting of the general conclusions is in regard to the 
phenomena of “attunement.” In several kinds of tropism, as the 
strength of stimulation is increased, a critical point is reached where 
the effect changes from positive to negative. This critical point 
differs in different species, and may be regarded as indicating an 
optimum intensity to which the organism is attuned. How is this 
attunement established? Natural selection is rejected and “a cause 
more consistent with sound physiology ” is sought. 
It has been shown that the effects of various agents persist after 
the stimulus has been removed. This “ after-effect ” seems to show 
that the agent causes a change in the protoplasm which is more or 
less permanent. This permits the accumulation of extremely slight 
effects. When, however, the repeated stimuli are each great, it is 
not an accumulation, but a diminution of response that is noticed. 
The organism becomes gradually accustomed to the stimulus and 
ceases to respond. It is acclimated. The author’s hypothesis to 
explain this is that the chemical change which leads to the response 
becomes a permanent characteristic of the protoplasm so that no 
