14 PLANT LIFE 
preserved it, but have acquired all the acces- 
sory complications of structure that purposive 
motion necessarily entails ? 
The answer is to be sought in the results of 
an apparently trivial difference in structure 
between the animals and plants which made 
its appearance at an early period in evolution- 
ary history. It is a difference which from 
the start was fraught with consequences of 
the greatest importance, and has profoundly 
affected the entire course of development 
in the two kingdoms respectively. Stated 
simply, it consists essentially in this, namely, 
that the living substance of the plant secretes 
over its surface a skin of cellulose, or some 
analogous substance, whilst that of the 
animal does not. 
If we examine any one of the simplest 
microscopic individuals of whose vegetable 
nature there can be no dispute, we shall find 
that the protoplasm, or living substance, is 
enclosed in a not-living skin or bag of cellulose. 
This skin is not an indispensable structure, 
for the living substance may, for a time at 
least, exist without it. Even in the highest 
plants this commonly occurs during the first 
stages of embryonic existence, but as soon 
as development begins the membrane is 
1 This statement is broadly true, for although cellulose 
is not unknown in the animal kingdom it has never been 
so arranged in the body as to affect the whole relations 
of the animal to its physical environment as it does in 
plants, 
