386 PHYSIOLOGICAL DEVELOPMENT. 



general result is still the same : the component molecules of 

 the tissue must have their molecular arrangements changed, 

 and the change in their molecular arrangements must go on 

 until their molecular motions are so re-adjusted as to equili- 

 brate the moh>cular motions of the new physico-chemical or 

 chemical agent. In other words, the organic matter com- 

 posing the part, if it continues to be organic matter at all, 

 must assume that molecular composition which enables it to 

 bear, or as we say adapts it to, the incident forces. 



Nor is it less certain that throughout the organism as a 

 whole, equilibration is alike the proximate limit of the changes 

 wrought by each action, as well as the ultimate limit of the 

 changes wrought by any recurrent actions or continuous 

 action. The ordinary movements every instant going on, are 

 movements towards a new state of equilibrium. Raising a 

 limb causes a simultaneous shifting of the centre of gravity, 

 and such altered tensions and pressures throughout the body 

 as re- adjust the disturbed balance. Passage of liquid into or 

 out of a tissue, implies some excess of force in one direction 

 there at work ; and ceases only when the force so diminishes or 

 the counter- forces so increase that the excess disappears. A 

 nervous discharge is reflected and re -reflected from part to 

 part, until it has all been used up in the re- arrangements pro- 

 duced — equilibrated by the reactions called out. And what 

 is thus obviously true of ever}^ normal change, is equally true 

 of every abnormal change — every disturbance of the estab- 

 lished rhythm of the functions. If such disturbance is a 

 single one, the perturbations set up by it, reverberating 

 throughout the system, leave its moving equilibrium slightly 

 altered. If the disturbance is repeated or persistent, its suc- 

 cessive efiects accumulate until they have produced a new 

 moving equilibrium adjusted to the new force. 



Each re-balancing of actions, having for its necessary con- 

 comitant a modification of tissues, it is an obvious corollary 

 that organisms subjected to successive changes of conditions, 

 must undergo successive differentiations and re-diff'erentia- 



