PHENOMENA OF LIFE 15 



slowly wastes away and the vital processes finally cease altogether. 

 These three periods, youth, maturity, and old age make up the 

 life cycle of every individual. 



(6) Reproduction. Perhaps the most remarkable charac- 

 teristic of a living organism is its ability to produce new indi- 

 viduals like itself at some time in the life cycle. This reproductive 

 power usually manifests itself during maturity. Among animals 

 an offspring may originate from its parent by asexual or sexual 

 methods. Reproduction may take place by a splitting of the 

 entire body into two or more parts, each of which takes up an 

 independent existence. This method of multiplication is often 

 called asexual reproduction; if the new individuals are formed by a 

 division of the original or parent body into approximately equal 

 parts, reproduction is said to be by fission; if, on the other hand, 

 only a small portion of the parent individual becomes separated 

 as a distinct organism, reproduction is by budding, and the off- 

 spring are called bads. Sometimes an animal passes through a 

 resting stage during which the body fragments into a large num- 

 ber of small parts which subsequently become free individuals. 

 This asexual method of reproduction is known as sporulation. 



Sexual reproduction involves the union of substances from two 

 different animals. When this process occurs in certain one-celled 

 animals, two individuals come together and exchange a portion 

 of their living substance (Fig. 31); in many-celled animals there 

 is a union of sexual cells, egg and sperm, which originate from 

 two separate individuals. Asexual reproduction increases the 

 number of individuals without altering the nature of their 

 substance; sexual reproduction originates new individuals by 

 mixing portions of two preexisting organisms. 



Asexual reproduction may be illustrated by Paramecium (p. 67), 

 a single-celled animal which divides twice during every twenty- 

 four hours when conditions are favorable. Its metabolism is so 

 rapid that it can grow to its maximum size in half that period of 

 time. The honey bee (Chap. XII) furnishes an excellent illus- 

 tration of sexual reproduction. Three sorts of bees exist: the 



