224 
Francis Darwin. 
geo-perception, and this, in spite of its barbarism, will, no doubt, be 
generally employed. 1 
I have taken the case of geo-perception as an instance of the 
classification of movement according to the nature of the stimulus. 
Pfeffer has suggested a fundamental application of this method in 
the new edition of his Physiology. He divides all reactions into 
autogenic and cetiogenic. Autogenic effects are those usually called 
spontaneous, such as the jerking of the leaflets of Desmodium. 
yEtiogenic actions are those performed at the suggestion of agents 
such as gravity, light, contact and the like. Klebs and other 
writers make the same division and describe actions as due either 
to external or internal stimulation. With all respect to the 
authority of Pfeffer 1 cannot believe it possible to uphold the 
distinction in all cases. The sleep-movements of leaves are due to 
the alternation of day and night, but when a plant continues to 
show these movements in darkness its behaviour is autogenic. 
Indeed it is hardly necessary to refer to a concrete case in order to 
show the difficulty of distinguishing sharply between auto- and 
cetiogenic reactions. Whenever a stimulus is repeated, its effect is 
due partly to that stimulus, and partly to the revival of one or more 
engrams which have become accessory internal stimuli. No doubt 
Pfeffer’s classification is necessary, all I would insist on is that the 
distinction between autogenic and cetiogenic actions is in many 
cases of less importance than the bond of association which unites 
them. 
The thesis of the “Power of Movement in Plants” is that the 
autogenic power of circumnutation is the basis from which the varied 
cetiogenic curvatures have been evolved, so that here again the two 
classes are assimilated rather than distinguished. 
We have seen that reactions may be classified according 
to the nature of the stimuli, but this is not the only available method. 
As far as we know, the mechanism of curvature is the same whether 
a seedling curves geogenically or photogenically : in the case of 
animals the movement is the same whether executed in response to 
a stimulus received through the eye or the ear,—and so it is with 
plants. Thus from this point of view photogenic and geogenic move¬ 
ments fall into a single class. Is it possible to sub-divide this class 
by other criteria? Pfeffer has shown that it is possible. He 
divides curvatures into the Tropic and Nastic which may be 
1 In an address to Section K, British Association, 1904, I sug¬ 
gested gravi-perception. This would be convenient, but the 
corresponding luci-perception is not likely to be adopted. 
