704 APPENDIX D. 



representable in terms of tlie mechanical hypothesis. Every 

 physicist will endorse the proposition that in each aggregate there 

 tends to establish itself an equilibrium between the forces exercised 

 by all the units upon each and by each upon all. Even in masses 

 of subsiance so rigid as iron and glass, there goes on a molecular 

 re-arrangement, slow or rapid according as circumstances facilitate, 

 which ends only when there is a complete balance between the actions 

 of the parts on the whole and the actions of the whole on the parts : 

 the implications being that every change in the form or size of the 

 whole, necessitates some redistribution of the parts. And though 

 in cases like these, there occurs only a polar re-arrangement of the 

 molecules, without changes in the molecules themselves ; yet where, 

 as often happens, there is a passage from the colloid to the crystal- 

 loid state, a change of constitution occurs in the molecules them- 

 selves. These truths are not limited to inorganic matter : they 

 unquestionably hold of organic matter. As certainly as molecules 

 of alum have a form of equilibrium, the octahedron, into which 

 they fall when the temperature of their solvent allows them to ag- 

 gregate, so certainly must organic molecules of each kind, no mat- 

 ter how complex, have a form of equilibrium in which, when they 

 aggregate, their complex forces are balanced — a form far less rigid 

 and definite, for the reason that they have far less definite polarities, 

 are far more unstable, and have their tendencies more easily modi- 

 fied by environing conditions. Equally certain is it that the special 

 molecules having a special organic structure as their form of 

 equilibrium, must be reacted upon by the total forces of this or- 

 ganic structure ; and that, if environing actions lead to any change 

 in this organic structure, these special molecules, or physiological 

 units, subject to a changed distribution of the total forces acting 

 upon them will undergo modification — modification which their 

 extreme plasticity will render easy. By this action and reaction 

 I conceive the physiological units peculiar to each kind of organ- 

 ism, to have been moulded along with the organism itself. Set- 

 ting out with the stage in which protein in minute aggregates, 

 took on those simplest differentiations which fitted it for differ- 

 ently-conditioned parts of its medium, there must have unceas- 

 ingly gone on perpetual re- adjustments of balance between aggre- 

 gates and their units — actions and reactions of the two, in which 

 the units tended ever to establish the typical form produced by 

 actions and reactions in all antecedent generations, while the 

 aggregate, if changed in form by change of surrounding condi- 

 tions, tended ever to impress on the units a corresponding change 

 of polarity, causing them in the next generation to reproduce the 

 changed form — their new form of equilibrium. 



This is the conception which I have sought to convey, though 

 it seems unsuccessfully, in the Principles of Biology ; and which I 

 have there used to interpret the many involved and mysterious 



