EXTRACT XLIV. B. 



ON MATERIO-DYNAMIC PARALLELISM, OR ORGANIC 

 CO-EVOLUTION AND -INVOLUTION OF TISSUE, 

 ORGAN, AND FUNCTION, AS A NORMAL MODE 

 OF DEVELOPMENT, OR GROWTH AND DECAY. 



WE take it, as a physiological rule, that the development 

 of structure, from the embryonic condition to the mature 

 state, is accompanied by the development, or evolution, 

 of function, in exact degree and proportion, or, in other 

 words, that the development of both is parallel or synchronous^ 

 and that when maturity is reached both are characterised 

 by absolutely normal, or co-ordinate, features and qualities, 

 enabling their possessor to take a normal place in the 

 battle of life, and to perpetuate the species, whatever it 

 may be ; any departure from this rule, therefore, intro- 

 duces an element of friction, material or dynamic, into 

 the physiological working and functional output of 

 texture, organ, or organism, which, if persistent, must 

 end in the production of a pathological condition or 

 disease. 



We contend that such an occurrence as the loss of this 

 parallelism may give rise to a condition of disease, pro- 

 portionate to the intensity and continuance of the cause, 

 or causes, and becomes an absolutely provable fact, when 

 we closely observe and consider the sequences of develop- 

 mental events occurring in the evolution and involution 

 of certain organs and structures, whose existence is 

 maintained for a limited period of time in the life of 

 the organism in which they occur, and whose functional 

 activity is only temporary, or within more or less brief 



