10 BIOLOGY 



food products. Fats contain the -three elements, carbon, oxygen, 

 and hydrogen, in this respect agreeing with the carbohydrates. 

 They are, however, considerably more complex than carbohy- 

 drates, a molecule of fat containing more atoms, as is shown by 

 the formula CsiHiwOg, which represents a common fat. When 

 treated by a simple chemical method, fats are broken up into two 

 substances, one of which is called glycerine and the other a fatty add. 



Sources of /ate. Fat can be manufactured by either ani- 

 mals or plants out of other foods. If an animal is fed upon 

 proteids or carbohydrates, it can manufacture fat from them; 

 and plants are able to make fat out of the food materials 

 which they absorb from the air and water. 



The table on page 11, which illustrates the composition of a 

 few of our common foods, shows that our ordinary diet con- 

 tains a fair proportion of each of these three foodstuffs. It 

 will also be seen from this table that the largest proportion 

 of proteids comes from animal foods, while the largest pro- 

 portion of carbohydrates comes from plant foods. 



ORIGIN OF LIFE 



Perhaps no feature of modern biology is more important 

 than the acceptance of the theory that every living thing 

 comes from a living source. All living animals and plants 

 with which we are familiar to-day have originated from pre- 

 viously existing life. The living animal comes from the egg 

 that was produced by another living animal; the plant comes 

 from a seed that was produced by another living plant. But 

 the question of the primal origin of life is sure to intrude itself 

 upon our minds, and we are forced to ask whether living things 

 can be, or ever have been produced by any other means. Did 

 there ever occur, or does there occur in the world to-day, a 

 spontaneous generation of life? In other words, did a living 

 thing ever arise from some source which was not alive? So 

 far as our knowledge of nature is concerned, there are no means 

 of starting new life except from previously existing life. 



