648 General and Applied Biology 



of cells and their more numerous tissues and organs must necessarily 

 have a more complicated nervous system than lower organisms in order 

 to ensure the proper coordination. Chemical and physical factors 

 probably play important roles in this respect. Could the digestive sys- 

 tem of an organism function properly if all the organs of that system 

 worked independently? How would one organ know what to do if it 

 were not properly notified what is expected of it? Another evidence 

 that all organs are closely related and associated is shown by the fact 

 that effects of illness in one part of a system are frequently relayed to 

 other parts of that system or even other systems so that the unity of 

 the organism as a whole may be again regained. In other words, "sym- 

 pathy" is expressed between the various units of a living organism. 

 This is quite essential. In some instances when defects or abnormalities 

 arise in a certain region, organs in some distant part of the animal 

 may take on extra responsibilities until the defect is remedied. If the 

 abnormality is not properly remedied, dissension may spread; over- 

 worked tissues cease to carry their double burdens and still greater con- 

 sequences result. In other words, a living organism is as healthy as its 

 weakest part. 



5. Unity Between the Various Systems of a Living Organism. — From 

 what has been said abo\e, it is quite evident that every living organism 

 must have the proper coordination and subordination of its various sys- 

 tems if efficiency is to result. A slight abnormality starting in a certain 

 tissue, unless corrected, may spread to other tissues, to organs, and to 

 systems, and eventually the organism as a whole may be affected. This 

 may appear to be a mistake in construction on the part of Nature, but 

 in reality it is a blessing in disguise, for without these consequences little 

 might be attempted to care for minor disturbances. Consequently, it 

 becomes highly desirable and essential to correct defects so that they do 

 not spread. 



B. Similarity of Structures and Functions Between Closely Related 

 Species of Organisms 



Students who are just beginning their study of biology will probably 

 have some difficulty in realizing the great number of similarities between 

 various apparently unrelated organisms. We have a great tendency to 

 observe one or two differences when comparing two organisms and not 

 to notice a greater number of similarities which are observ^ed only after 

 detailed study. As you study the structures, functions, and reactions of 

 various types of living organisms, you will observe many more similari- 



