100 THE STORY OF GERM LIFE. 



of plant life consist of such materials as sugar, 

 starches, fats, and proteids, all of which have 

 been manufactured by the plant from the ingre- 

 dients furnished it from the soil and air, and 

 through the agency of the sun's rays. These 

 products of plant life now form foods for the 

 animal kingdom. Starches, fats, and proteids are 

 animal foods, and upon such complex bodies 

 alone can the animal kingdom be fed. Animal 

 life, standing high up in the circle, is not capable 

 of extracting its nutriment from the soil, but must 

 take the more complex foods which have been 

 manufactured by plant life. These complex 

 foods enter now into the animal and take their 

 place in the animal body. By the animal activi- 

 ties, some of the foods are at once decomposed 

 into carbonic acid and water, which, being dis- 

 sipated into the air, are brought back at once 

 into the condition in which they can serve again 

 as plant food. This part of the food is thus 

 brought back again to the bottom of the circle 

 (Fig. 25, dotted lines). But while it is true that 

 animals do thus reduce some of their foods to 

 the simple condition of carbonic acid and water, 

 this is not true of most of the foods which con- 

 tain nitrogen. The nitrogenous foods are as 

 necessary for the life as the carbon foods, and 

 animals do not reduce their nitrogenous foods 

 to the condition in which plants can prey upon 

 them. While plants furnish them with nitroge- 

 nous food, they can not give it back to the plants. 

 Part of the nitrogenous foods animals build into 

 new albumins (Fig. 25 C); but a part of them they 

 reduce at once into a somewhat simpler condition 

 known as urea. Urea is the form in which the 

 nitrogen is commonly excreted from the animal 



