26 THE DATA OF BIOLOGY. 



stituent molecules or atoms; since, in proportion as an inci- 

 dent force impresses but little motion on a mass, it is the 

 better able to impress motion on the parts of the mass in 

 relation to one another. And it is further probable that the 

 extreme contrasts in molecular mobilities among the compo- 

 nents of these highly complex molecules, aid in producing 

 modifiability of arrangement among them. 



Lastly, the great difference in diffusibility between colloids 

 and crystalloids, makes possible in the tissues of organisms 

 a specially rapid re-distribution of matter and motion; both 

 because colloids, being easily permeable by crystalloids, can 

 be chemically acted on throughout their whole masses, in- 

 stead of only on their surfaces; and because the products of 

 decomposition, being also crystalloids, can escape as fast as 

 they are produced : leaving room for further transformations. 

 So that while the composite molecules of which organic tis- 

 sues are built up, possess that low molecular mobility fitting 

 them for plastic purposes, it results from the extreme 

 molecular mobilities of their ultimate constituents, that the 

 waste products of vital activity escape as fast as they are 

 formed. 



To all which add that the state of warmth, or increased 

 molecular vibration, in which all the higher organisms are 

 kept, increases these various facilities for re-distribution : not 

 only as aiding chemical changes, but as accelerating the 

 diffusion of crystalloid substances. 



