356 PHYSIOLOGICAL DEVELOPMENT. 



perty which protoplasm possesses in a lower degree, is mani- 

 fest. The sarcode of a Rhizopod and the substance of an 

 unimpregnated ovum, exhibit movements that imply a pro- 

 pagation of stimulus from one part of the mass to another. 

 We have not far to seek for a probable origin of this pheno- 

 menon. There is good reason for ascribing it to the extreme 

 instability of the organic colloids of which protoplasm con- 

 sists. These, in common with colloids in general, assume dif- 

 ferent isomeric forms with great facility ; and they display not 

 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 1 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 



