THE BELFAST ADDRESS. 183 



two factors there is incessant interaction. The organism 

 is played upon by the environment, and is modified to 

 meet the requirements of the environment. Life he de- 

 fines to be 'a continuous adjustment of internal relations 

 to external relations.' 



In the lowest organisms we have a kind of tactual 

 sense diffused over the entire body ; then, through im- 

 pressions from without and their corresponding adjust- 

 ments, special portions of the surface become more 

 responsive to stimuli than others. The senses are 

 nascent, the basis of all of them being that simple 

 tactual sense which the sage Dernocritus recognised 

 2,300 years ago as their common progenitor. The 

 action of light, in the first instance, appears to be a 

 mere disturbance of the chemical processes in the animal 

 organism, similar to that which occurs in the leaves of 

 plants. By degrees the action becomes localised in a 

 few pigment-cells, more sensitive to light than the 

 surrounding tissue. The eye is incipient. At first it is 

 merely capable of revealing differences of light and 

 shade produced by bodies close at hand. Followed, as 

 the interception of the light commonly is, by the con- 

 tact of the closely adjacent opaque body, sight in this 

 condition becomes a kind of ' anticipatory touch.' The 

 adjustment continues ; a slight bulging out of the 

 epidermis over the pigment-granules supervenes. A 

 lens is incipient, and, through the operation of infinite 

 adjustments, at length reaches the perfection that it 

 displays in the hawk and eagle. So of the other senses ; 

 they are special differentiations of a tissue which was 

 originally vaguely sensitive all over. 



With the development of the senses, the adjustments 

 between the organism and its environment gradually 

 extend in space, a multiplication of experiences and a 

 corresponding modification of conduct being the result. 



