358 PHYSIOLOGICAL DEVELOPMENT. 



are apt to communicate a like change of form to ad- 

 jacent molecules of the same kind — the impact of each 

 overthrow is passed on and produces another overthrow. 

 Probably the proneness towards isochronism of molecular 

 movements necessitates this. If any molecule has had 

 its components re-arranged, and their oscillations conse- 

 quently altered, there result movements not concordant with 

 the movements in adjacent untransformed molecules, but 

 which, impressing themselves on the parts of such untrans- 

 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 

 molecules 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 



