364 Appendix 
reacts negatively toward a falling but not toward a rising 
temperature. We see also that Euglena in a moderate 
light reacts negatively toward a decrease but not toward 
an increase in the intensity of light, whereas in a stronger 
light the reaction is exactly reversed.* 
The tendency of organisms to maintain their physio- 
logical state unaltered consequently resolves itself into a 
tendency to invariability in their external and internal 
environments. Thus for instance, oysters and actinians 
close when exposed to the air; that is, they behave so as 
to keep the standard of moisture unaltered within them- 
selves and in their immediate surroundings.* 
To the tendency toward invariability of environment 
is due also the position which the organism takes with 
relation to the direction of the various forces to which 
it is exposed, especially gravity. Hence the tendency to 
preserve or restore its normal position. Thus, for in- 
stance, the amceba draws in its pseudopodia when they 
come in contact with solid non-edible bodies; but if it is 
lifted off the bottom of the aquarium and is suspended 
in the water it stretches out its pseudopodia in all direc- 
tions. As soon as one of these touches a solid object, 
the amceba takes hold of it, draws its body over to it, and 
again resumes its original position. Likewise a starfish 
when inverted tries to turn, over, that is, to return to 
its normal environmental conditions with relation to 
gravity.® 
All “needs”to throw off substances which have been 
produced by the general metabolism and which the or- 
3Jennings, Behavior of Lower Organisms, pp. 294-205. 
4H. Piéron, L’évolution de la mémoire, pp. 29, 74. Paris, Flam- 
marion, I9Io. 
5K. C. Schneider, Vorlesungen iiber Tierpsychologie, pp. 5, 57. 
Leipsic, Engelmann, 1909. 
