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 tlie 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, i.e. certain Tunicates, and very many 

 plants like the Fungi, and many parasites and sapro- 



