IOWA ACADEMY OF SCIENCES. 
49 
of protoplasm. Mathews would find the basis of conduc- 
tivity in a wave of gelation in the colloids of irritable 
tissue. 
“Taxis’’ or “tropism” in its several forms is today seen 
to result from the unequal stimulation of a cell at its 
opposite poles. The minuteness of the possible differences 
between the strength of stimuli at the two extremities of 
a cell is best seen in chemotaxis. But however small this 
difference, if it be sufficient to affect the transformation of 
energy at the pole proximal to the stimulus, unilateral 
movement must result. Hence the wonderfully fascinating 
directive effects of external agencies upon the lower 
organisms rest upon local differences of chemical reaction 
in the cell. 
Work on the motile Infusoria has indicated in a very 
striking way the nature of a reflex. Whatever the form of 
stimulus, — mechanical, thermal, chemical, electrical, — the 
same reflex is exhibited. This reflex is itself invariable, 
embracing an arrest of the forward progression, a swim- 
ming backward, a turning toward a structurally defined 
side, then a swimming forward. Such a reflex is entirely 
effective under the usual conditions for the life of the 
animal, because the anterior extremity first comes in con- 
tact with sources of danger, and the series of movements 
leads to the danger being avoided. 
An insight into adaptation to environment has been 
given by the fine work of Jennings upon fixed Infusoria, 
particularly Stentor. Here it was shown that protoplasm 
has results wrought into it from the previous experiences 
to which the organism has been subjected. The behavior 
of Stentor during the course of repeated stimulation soon 
begins to show traces of after-effects from the stimuli 
which have just preceded. Such adaptation to conditions 
must have its basis sought in alterations produced in the 
architecture and chemistry of protoplasm under the impact 
of energy from without, the modification of structure per- 
sisting for a greater or less time. The field is certainly a 
fruitful one for further and more refined investigation. 
