Form and Color in Nature. 241 



remain attached, and the individual becomes a congeries. 

 Affected by forces upon every side, a differentiation gradually 

 takes place ; certain portions of the surface become firmer, 

 others softer ; food is taken in at certain particular points, or 

 at one point alone a division of function has arisen. A 

 distinction has also arisen between the animal and the 

 vegetable, and with this distinction comes a difference in 

 constitution, effecting a distinction in color. The vegetable 

 forms contain an ingredient called chlorophyl which re- 

 flects the green rays ; the animal tissue reflects the brown 

 and gray. I speak, of course, in general terms. 



Before these changes have taken place and while they 

 are proceeding, we notice that sensation has arisen that 

 is, a response to external impressions. And here let me 

 pause to call attention to the nature of physical impressions, 

 or sensations, as we know them. We speak of our five 

 senses : of sight, of hearing, of touch, of taste, and of smell, 

 not meaning that these terms indicate all the impressions 

 which we perceive, but merely using a traditional expres- 

 sion describing certain obvious classes. All these are the 

 result of different qualities and quantities of motion. Now, 

 it is extremely difficult, perhaps impossible, for us to pass 

 back through the various forms of life and tell with any 

 certainty just when any particular sense had its beginning, 

 or what was the nature of that beginning. We can only 

 perceive these things when they have acquired a certain 

 definiteness and stability. Nevertheless, we can with some 

 degree of clearness trace their development, and can, I think, 

 safely assume that the first of them which was acquired was 

 touch, or that mere susceptibility to impression in general 

 which, in default of a better term, we may entitle touch. 

 I think it is evident also that whatever sensitiveness ex- 

 isted at the outset was equally present in all parts of the 

 individual. 



But as the unicellular individual gave rise to the aggre- 

 gation of such, and as in this aggregation the individuals 

 gradually surrendered their independence and assumed spe- 

 cific functions, in the way of surface protection, of the 

 management of the commissariat the receipt and disposi- 

 tion of food ; as the avoidance of obstacles and the search 

 for sustenance gradually gave definiteness and direction to 

 motion, the sensations of different parts became gradually 

 differentiated. It would be natural that is, in accordance 

 with experience to expect that there would be a special 



