CHAP, III.] OF SENSE ORGANS. 57 



organisms are, however, often affected quite differently by 

 this agent. They may move away from rather than 

 towards its source, and to this extent may be said to 

 ' seek ' the shade rather than the glare of sunlight. 



The operation of such influences and their results, 

 form the beginnings or substrata, as it were, of other 

 phenomena with which we are now more particularly 

 concerned. The unilateral influence of Light and the 

 movements to or from its source to which it gives rise, 

 afford a connecting link between diffused causes like 

 Heat, which operate generally and produce purely random 

 motions, and those more localized influences now to be 

 considered, by which, and the intermediation of a more 

 and more complex Nervous System, the various definite 

 or responsive movements of organisms have been gradually 

 evoked and potentially organized. 



Touch. The first to be considered because it is the 

 simplest of these localized influences, is a shock on me- 

 chanical impact of some kind falling upon the external 

 surface of the organism. This is the primordial and most 

 general of all the modes by which the surface of an 

 organism is impressible. Its sensitivity to such stimuli 

 is both in the stage of impression and in that of 

 reaction closely akin to the general organic irritability of 

 protoplasm, which unquestionably constitutes its starting 

 point. These modes of impression and reaction are the 

 first links towards the establishment of a correspondence 

 between the organism and the most common events or 

 properties of the medium in which it lives and moves. 

 It is, consequently, the kind of impressibility most exten- 

 sively called into play in all the lower forms of animal 

 life. 



Although the whole or the greater part of the surface of 



