8 INTRODUCTION 



of the prime necessities for the continued existence of 

 life on earth. Otherwise the earth would shortly become 

 completely covered with deatl bodies. The raw materials 

 which are combined by the green plants must eventually 

 be returned to the air and soil. Although these decom- 

 positions are chemical reactions, like those occurring in 

 living bodies, they do not occur spontaneously any more 

 than water and carbon dioxid will spontaneously form 

 sugar. In both cases the presence of living organisms 

 is necessary under ordinary natural conditions. Decom- 

 position is brought about by small non-green plants 

 chiefly, although animals in liberating energy for doing 

 work return large quantities of water and carbon dioxid 

 to the soil and air. The proper treatment of soils to 

 secure the most desirable growth of these small plants 

 and the maximum enrichment of the soil through their 

 action forms a most interesting chapter in modern 

 achievement. 



8. The Reproduction of Plants and Animals is just 

 as necessary as their ability to live and maintain them- 

 selves. The means by which this is attained are most 

 varied, interesting, and important. In fact they are so 

 diverse that they can be treated in a book of this sort 

 only in a most general way. 



9. Heredity. — Although so familiar as to have lost 

 its novelty, it is nevertheless a most curious and interest- 

 ing fact that offspring closely resemble their parents but 

 are never exactly like them. This is said to be due to 

 heredity. Although our knowledge of the laws of he- 

 redity has been acquired almost wholly in the last twenty 

 years, the last ten of them have seen more rapid advance 

 in the understanding of the mechanism of heredity than 

 in almost any other line of biological investigation. Not 

 alone from its human and scientific interest but also on 

 account of its great practical utility (in breeding economic 

 plants and animals) does this subject deserve discussion 

 in this course. 



