36 THE DATA OF BIOLOGY. 



matter called chloroph}^!!, which gives the green colour to 

 leaves, makes its appearance whenever the blanched shoots 

 of plants are exposed to the Sun; that the petals of flowers, 

 uncoloured while in the bud, acquire their bright tints as 

 they unfold; and that on the outer surfaces of animals, 

 analogous changes are induced; are v/ide inductions which 

 are enough for our present purpose. 



§ 14. We come next to the agency of chief importance 

 among those that work changes in organic matter; namely, 

 chemical affinity. How readily vegetal and animal substances 

 are modified by other substances put in contact with them, 

 we see daily illustrated. Besides the many compounds which 

 cause the death of an organism into which they are put, we 

 have the much greater number of compounds which work 

 those milder effects termed medicinal — effects implying, like 

 the others, molecular re-arrangements. Indeed, most soluble 

 chemical compounds, natural and artificial, produce, when 

 taken into the body, alterations that are more or less mani- 

 fest in their results. 



After what was shown in the last chapter, it will be mani- 

 fest that this extreme modifiability of organic matter by 

 chemical agencies, is the chief cause of that active molecular 

 re-arrangement which organisms, and especially animal 

 organisms, display. In the two fundamental functions of 

 nutrition and respiration, we have the means by which the 

 supply of materials for this active molecular re-arrangement 

 is maintained. 



The process of animal nutrition consists partly in the ab- 

 sorption of those complex substances which are thus highly 

 capable of being chemically altered, and partly in the absorp- 

 tion of simpler substances capable of chemically altering them. 

 The tissues always contain small quantities of alkaline and 

 earthy salts, which enter the system in one form and are 

 excreted in another. Though we do not know specifically 

 the parts which these salts play, yet from their universal 



