360 TPHYSIOLOGICAL DEVELOPMENT. 



duced. We know that in individual men and animals, the 

 power of sustained action in muscles is rapidly adaptable to 

 the amount of sustained action required. We know that 

 being "out of condition, " is usually less shown by the inability 

 to put out a violent effort than by the inability to continue 

 making violent elForts ; and we know that the result of train- 

 ing for prize-fights and races, is more shown in the prolonga- 

 tion of energy than in the intensification of energy. At the 

 same time, experience has taught us that the structural change 

 which accompanies this functional change, is not so much a 

 change in the bulk of the muscles as a change in their inter- 

 nal state : instead of being soft and flabby they become hard. 

 We have inductive proof, then, that exercise of a muscle causes 

 some interstitial growth along with the power of more sus- 

 tained action ; and there can be no doubt that the one is a 

 condition to the other. What is this interstitial growth ? 

 There is reason to suspect that it is in part an increased 

 deposit of the sarcous substance and in part a development of 

 blood-vessels. Microscopic observation tends to confirm the 

 conclusions before drawn, that repetition of contractions fur- 

 thers the formation of the matter which contracts, and that 

 greater draughts of blood determine greater vascularity. 

 And if the contrasts of molecular structure and the contrasts 

 of vascularity, directly caused in muscles by contrasts in their 

 activities, are to any degree inheritable ; there results an 

 explanation of those constitutional difierences in the colours 

 and textures of muscles, which accompany constitutional 

 differences in their degrees of activity. 



It may be added that if we are warranted in so ascribing 

 the difiFerentiations of muscles from one another to direct 

 equilibration, then we have the more reason for thinking 

 that the differentiation of muscles in general from other 

 structures is also due to direct equilibration. That unlike- 

 nesses between parts of the contractile tissues having unlike 

 functions, are caused by the unlikenesses of their functions, 

 renders it the more probable that the unlikenesses between 



