FOOD OF PLANTS AND ANIMALS. 155 



worm is destructive, and dissipates energy. And this difference 

 becomes of immense importance in view of the fact that the 

 fern is typical in this respect of all green plants, as the earth- 

 worm is typical of all animals. 



It will hereafter api)ear that even this difference, great as it 

 is, is partly bridged over by colorless plants like yeast, nionlds, 

 bacteria, etc., which have no chlorophyll, are therefore unable 

 to use the energy of light, and hence must have energized fo(>d. 

 But these organisms do not, like animals, require proteid food, 

 being able to extract all needful energy from the simpler fats, 

 carbohydrates, and even from certain salts. When we consider 

 that the distinctive peculiarities of animals can thus be reduces I 

 to the sole characteristic of dependence on proteid food, we can- 

 not doubt that the differences between plants and animals are of 

 immeasurably less importance than their fundamental likeness. 



It has been the object of the foregoing chapters to give the 

 student a general conception of organisms, whether vegetal or 

 animal ; of their structure, growth, and mode of action ; of their 

 position in the world of matter and energy, and of their relations 

 to lifeless things. With this preliminary knowledge as a basis, 

 the student is prepared to take up the progressive study of other 

 organisms, selected as convenient types or examples. It is con- 

 venient to begin with low and simple forms of life and work 

 gradually upwards; and it is especially desirable to do so be 

 cause there is reason to believe that this course corresponds 

 broadly with the path of actual evolution. 



