290 HIGH SCHOOL ZOOLOGY. 



diseases of cultivated plants and animals are frequently avert 

 ible by a proper knowledge of the life-history of the organisms 

 in question. Many of these diseases are merely phases of the 

 struggle for existence, which occurs not only between individuals 

 of the same species but between species of the most different 

 character. Other diseases ai"e due to unfavorable surroundins: 

 conditions, which a due attention to physiology may make it 

 posssible to remedy. Such disturbances of the normal or 

 healthy pi-ocesses of life form the subject-matter of Vegetable 

 and Animal Pathology. 



For further information on these as well as on the other 

 topics discussed in this chapter, the student must, however, 

 consult books which deal with them specially. The object of 

 the })resent chapter has chiefly been to show the relations of 

 facts scattered throughout the preceding chapter-', and to indi- 

 cate as far as possible the general principles which have been 

 reached by zoologists. 



