THE INNER TISSUES OF ANIMALS. 347 



simply isomerism but polymerism. Further, this readiness to 

 undergo molecular re-arrangement, habitually shows itself in 

 colloids by the rapid propagation of the re-arrangement 

 from part to part. As Prof. Graham has shown, matter in 

 this state often " pectizes " almost instantaneously a touch 

 will transform an entire mass. That is to say, the change of 

 molecular state once set up at one end, spreads to the other 

 end there is a progress of a stimulus to change ; and this is 

 what we see in a nerve. So much being understood, let us 

 re-state the case more completely. 



Molecular change, implying as it does motion of molecules, 

 communicates motion to adjacent molecules ; be they of the 

 same kind or of a different kind. If the adjacent molecules, 

 either of the same kind or of a different kind, be stable in 

 composition, a temporary increase of oscillation in them as 

 wholes, or in their parts, may be the only result ; but if they 

 are unstable there are apt to arise changes of arrangement 

 among them, or among their parts, of more or less permanent 

 kinds. Especially is this so with the complex molecules 

 which form colloidal matter, and with the organic colloids 

 above all. Hence it is to be inferred that a molecular dis- 

 turbance in any part of a living animal, set up by either an 

 external or internal agency, will almost certainly disturb and 

 change some of the surrounding colloids not originally im- 

 plicated will diffuse a wave of change towards other parts 

 of the organism : a wave which will, in the absence of per- 

 fect homogeneity, travel further in some directions than in 

 others. Let us ask next what will determine the 



differences of distance travelled in different directions. Ob- 

 viously any molecular agitation spreading from a centre, will 

 go furthest along routes that offer least resistance. What routes 

 will these be ? Those along which there lie most molecules 

 tfiat are easily changed by the diffused molecular motion, and 

 which yet do not take up much molecular motion in assuming 

 their new states. Molecules which are tolerably stable will 

 cot readily propagate the agitation ; for they will absorb it 



