48? 



represcntable in terms of the 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 substance 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 

 implication 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 crystalloid state, 

 a change of constitution occurs in the molecules themselves. 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 aggregate, so certainly 

 must organic molecules of each kind, no matter 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 un- 

 stable, and have their tendencies more easily modified 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 organic 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 physio- 

 logical units peculiar to each kind of organism, to have been 

 moulded along with the organism itself. Setting out with 

 the stage in which protein in minute aggregates, took on 

 those simplest differentiations which fitted it for differently- 

 conditioned parts of its medium, there must have unceasingly 

 gone on perpetual re-adjustments of balance between aggregates 

 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 conditions, 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 



