THE INNER TISSUES OF ANIMALS. 349 



formed molecules, tend to generate in them concordant move- 

 ments — tend, that is, to produce the re-arrangements involved 

 by these concordant movements. Is this action limited to 

 strictly isomeric substances ? or may it extend to substances 

 that are closely allied? If along with the molecules of a 

 compound colloid there are mingled those of some kindred 

 colloid ; or if with the molecules of this compound colloid 

 there are mingled the components out of which other such 

 molecules may be formed ; then there arises the question — 

 does the same influence which tends to propagate the iso 

 meric transformations, tend also to form new molecules of 

 the same kind out of the adjacent components ? There is 

 reason to suspect that it does. Already when treating of the 

 nutrition of parts (§ 64), it was pointed out that we are obliged 

 to recognize a power possessed by each tissue to build up, out 

 of the materials brought to it, molecules of the same type as 

 those of which it is formed. This building up of like mole- 

 cules seems explicable as caused by the tendency of the 

 new components which the blood supplies, to acquire move- 

 ments isochronous with those of the like components in the 

 tissue ; which they can do only by uniting into like com- 

 pound molecules. Necessarily they must gravitate towards a 

 state of equilibrium ; such state of equilibrium — moving 

 equilibrium of course — must be one in which they oscillate 

 in the same times with neighbouring molecules ; and so 

 to oscillate they must fall into groups identical with the 

 groups around them. If this be a general principle of 

 tissue- growth and repair, we may conclude that it will apply 

 in the case before us. A wave of molecular disturbance 

 passing along a tract of mingled colloids closely allied in com- 

 position, and isomerically transforming the molecules of one 

 of them, will be apt at the same time to form some new mole- 

 cules of the same type, at any place where there exist the 

 proximate components, either uncombined or feebly combined 

 in some not very different way. And this will be most likely 

 to occur where the molecules of the colloid that are under- 



