78 FOOT-NOTES TO EVOLUTION. 
were no break in the uniformity of the environment, 
there would be no need of such knowledge. If there 
were no variation in lights and shadows, the eye would 
be powerless to bring information. The senses deal 
with changes or breaks in reality rather than with reali- 
ties themselves. Because, in action, the organism must 
be obedient to the demands of its environment, it is the 
function of the eye to make known these demands. 
The existence of the eye is therefore a concession to the 
environment. A concession of like nature is the brain 
itself, of which the eye and the sense organs in general 
may be considered as prolongations. These appendages 
of the brain carry to it truth of varying kind or degree. 
This truth as to external nature furnishes the basis of 
that obedience which in the animal expresses itself in 
action. 
The respiratory apparatus is an adaptation for the 
purpose of purifying the blood from the waste produced 
in the processes of life. It is a concession on the one 
hand to the demands of life in cell and tissue, and on 
the other hand to the nature of the surrounding medium. 
A change in the atmosphere would demand a correspond- 
ing change in the organs of breathing. If such a con- 
cession were impossible, the species in question would 
become extinct, as its individuals would perish. If the 
concessions necessary to continued existence should in- 
volve changes in other organs, the process of the sur- 
vival of the obedient would in time produce these 
changes. 
If there were no surrounding medium there would be 
no organ of respiration. If there were no light there 
would be no organ of vision. If there were no sound 
there would be no ear. If there were no motion there 
would be no need for knowledge, and therefore no sen- 
sation. If there were no power of locomotion there 
