34 THE DATA OF BIOLOGY. 



those milder effects termed medicinal— effects implyingj like 

 the others, molecular re-arrangements. Indeed, nearly all 

 soluble chemical compounds, natural and artificial, produce, 

 when taken into the body, alterations that are more or less 

 conspicuous 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 or- 

 ganisms, 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. 



Thus the process of animal nutrition consists in the absorp- 

 tion, partly of those complex substances that 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 alka- 

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

 and are excreted in another. Though we do not know spe- 

 cifically the parts which these salts play, yet from their 

 universal presence, and from the transformations which they 

 undergo in the body, it may be safely inferred that their 

 chemical afiinities are instrumental in working some of the 

 metamorphoses ever going on. 



The inorganic substance, however, on which mainly depend 

 these metamorphoses in organic matter, is not swallowed 

 along with the solid and liquid food, but is absorbed from 

 the surrounding medium — air or water, as the case may be. 

 Whether the oxygen taken in, either, as by the lowest 

 animals, through the general surface, or, as by the higher 

 animals, through respiratory organs, is the immediate cause 

 of those molecular changes that are ever going on through- 

 out the living tissues ; or whether the oxygen, playing the 

 part of scavenger, merely aids these changes by carrying 

 away the products of decompositions otherwise caused; it 



