INTRODUCTION 11 
sibility of making any absolute separation between 
them. ! 
A popular misconception of the province of biology 
assigns to it only a study of animal functions ; but the 
scientific biologist recognizes the fundamental likeness 
in the structure and functions of plants and animals, 
and realizes that any complete survey of the science 
must take equal cognizance of both. 
For practical purposes, inasmuch as all but the lowest 
forms of life are readily to be assigned to one kingdom 
or the other, it is desirable to retain the old divisions 
of zoology and botany; but this does not imply any 
absolute differences between the two great divisions of 
living things. The popular belief that plants and ani- 
mals differ essentially in their life-processes is errone- 
ous. Plants feed, breathe, and reproduce, exactly as do 
animals. It is true that the green cells of plants are 
able to absorb carbon dioxide from the atmosphere, and 
to utilize it in the manufacture of carbon compounds, 
a power, which, so far as we know, is lacking in ani- 
mals. This assimilation—or as it has been better 
termed “ photo-synthesis ”-—is not to be confounded 
with respiration, which takes place in all plants pre- 
cisely as in animals, but, being less energetic, is 
masked by the evolution of an excess of oxygen in 
those green cells which are exposed to sunlight. This 
photo-synthesis, and the character of the cell-wall, 
which in young plant cells is always composed of the 
carbo-hydrate cellulose, are the most marked charac- 
teristics of ordinary plants; but as cellulose occurs in 
some animals, z.e. certain Tunicates, and very many 
plants like the Fungi, and many parasites and sapro- 
