THE LIVING SUBSTANCE 5 



pathology are both of great importance in modern agri- 

 culture, the one to provide new and improved sorts of 

 plants and the other to find means of checking the enor- 

 mous losses due to the attacks of parasitic fungi and 

 bacteria on crop plants. A list of this sort might be 

 greatly extended, but it is probably already clear that 

 modern Biology has become a vast and complicated 

 subject, one in which the ordinary citizen can hope to 

 know only the general facts and principles. 



General Biology aims to bring together and present 

 to the non-specialist those aspects of Biology which it is 

 important that everyone should understand in order to 

 perform his duty as a citizen in an intelligent manner. 

 Whereas the professional biologist must limit his field 

 and specialize in order to keep the range of investigation 

 within such bounds as will permit him to become a 

 master of all that is known concerning it, the general 

 biologist strives to bring together the most important 

 facts and principles and so relate them that the average 

 person can usefully apply them to his ordinary affairs. 

 In an elementary course many interesting and important 

 subjects must be omitted or curtailed because they are 

 too difficult, or the student's previous training has been 

 inadequate, or because the time is not sufficient. In 

 conformity with these limitations the following subjects 

 have been chosen for consideration in the belief that they 

 are of such importance that no educated person can 

 afford to be ignorant of them: 



1. The Living Substance, Protoplasm. — It will be 

 shown that every living thing is alive because it is com- 

 posed in whole or part of a peculiar substance called 

 protoplasm. Furthermore this protoplasm is not only 

 of universal occurrence but it is very similar even in the 

 most diverse organisms. It is certainly worth our while 

 to understand the physical, chemical, and physiological 

 characteristics of this most wonderful of all substances. 



2. Green Plants play a peculiar and most important 



