8 EASY LESSONS IN VEGETABLE BIOLOGY. 



the parts of which they are composed, their relations 

 to each other, and the uses which they serve. It takes 

 in the entire life-history of every living thing, with 

 the changes which occur in health and disease. If 

 fully recorded, the world would hardly hold the books 

 which might be written about these things, since there 

 are many thousands of species, or different kinds, 

 both animal and vegetable ; and the influences to 

 which they are subject are quite innumerable. Yet 

 many observations and comparisons have shown that 

 one kind agrees with another in certain particulars, 

 so that the general principles which underlie their 

 forms, changes, structure, and uses may be under- 

 stood. The consideration of these general principles 

 and correspondences makes up the chief part of the 

 study of Biology. 



3. Biology studies only living beings. The general 

 forces of nature and the changes in non-living matter 

 are the subjects of Physics and Chemistry. Biology 

 only refers to these changes as they affect living 

 things, or are modified by the presence of life. 



4. Those astronomers who still hold to the Nebular 

 hypothesis the theory that the sun and planets de- 

 veloped themselves out of a sort of fiery vapor or 

 nebulous matter teach us that only a few of the 

 planets of our solar system are capable of sustaining 

 life, and chemical analysis shows that only four out 

 of nearly seventy elementary or simple substances, of 



