THE EVOLUTION OF THE MIND. 257 
should say that function precedes the differentiation of 
the organ on which it depends. There is a certain work 
to be done and a certain body of cells are set apart 
sooner or later to do it. Just as plough- 
Function ing was done in some fashion before the 
precedes A . fitheai h i 
Sanur invention of the plough, so in some man- 
ner respiration was accomplished before 
the development of gills and lungs. Something of men- 
tal action came before there was an organized brain. 
This law involves nothing mysterious or incomprehen- 
sible. It does not, so far as we know, imply the pre- 
existence of mind or the carrying out of any predeter- 
mined purpose in development. All this may be or may 
not be, but the phenomena in question throw no light 
on it. The fact seems to be that when the bodily pro- 
cesses make certain demands on an organism, these de- 
mands will be met in some fashion. Through natural 
selection some better structure will come into competi- 
tion. The cells and tissues on which the function de- 
pends will be specialized as an organ. In creatures of 
different ancestry the same function may be discharged 
by widely different organs. Conversely, what is ances- 
trally the same organ may in different groups of ani- 
mals serve functions widely different. 
In the animals of one cell, or protozoa, breathing 
and digestion are each performed by the whole body. 
In the division of labour or specialization which arises 
in the higher or many-celled animals certain alliances of 
cells or tissues are set apart for respiration alone, and 
certain others for digestion, while other functions of 
animal life are relegated to still other cell alliances. 
Each organ in turn is released from all functions except 
its own. 
Irritability, or the response to external stimulus, is 
an attribute of all living organisms. In the method 
