32 ORGANIC BEHAVIOUR 
“the stimulus supplied by continuous pressure that is so 
delicately perceived, while the stimulus supplied by dmpact is 
disregarded.” * And, comparing this with what is observed 
in the Venus’s Fly Trap, he says: “In these two plants the 
power of discriminating between these two kinds of stimuli 
has been developed to an equally astonishing extent, but in 
opposite directions.” t It is well, however, to avoid terms 
which carry with them so distinctively a conscious implication 
as “ discrimination ” and “ perception ” do for most of us. Just 
as the photographer’s film reacts differently according to the 
quality of light-rays, violet or red, which reach it, so do many 
organic substances react differently to stimuli of different 
quality, irrespective of their intensity. The “ discrimination ” 
of plants and of some of the lower animals is of this kind, 
and it is better to speak of it simply as differential reaction. 
There can then be no chance of its being confused with con- 
scious choice. 
Nor should the movements of the Sun-dew tentacles or of 
those of the Sea-~anemone be termed in strictness reflex action. 
As originally employed by Marshall Hall, and, since that time, 
by common consent, reflex action involves a differentiated 
nervous system. There is, first, an afferent impulse from 
the point of stimulation passing inwards to a nerve-centre ; 
secondly, certain little-understood changes within this centre ; 
and thirdly, an efferent impulse from the centre to some organ 
or group of cells which are thus affected. In plants there is 
no indication of anything analogous to this specialized mode 
of response. The impulse passes directly from the point of 
stimulation to the part affected without the intervention of 
anything like a nerve-centre. In the sensitive Oxalis the 
impulse passes directly to the point of insertion of the leaflet 
or leaf-stalk ; in Catasetum, from the horn to the retaining 
membrane; in the Sun-dew, from the affected tentacle to 
those in its neighbourhood. Even in the Sea-anemone, 
though there is a loosely diffused nervous system, the passage 
* “Mental Evolution in Animals,” p. 50, 
+ Ibid., p. 51. 
