20 THE USES AND OFvIGIN 



words, the occurrence of actions of any kind in living 

 matter, tends to occasion structural changes therein. 

 Such a fact is implied in the common statement that 

 living matter is an organizable matter. We suppose 

 nothing unusual, therefore, when we imagine that 

 frequently recurring contractions in any one portion of 

 living protoplasm will almost certainly lead to a structural 

 change therein. And, further, we are warranted in 

 supposing that such structural change will be of a kind to 

 favour the occurrence of the actions by which it has itself 

 been produced — that is, that the modified protoplasm will 

 be more highly contractile than the original protoplasm 

 from which it has been produced. 



But what, it may be asked, is the cause of these locally 

 recurring contractions, the occurrence of which is supposed 

 eventually to lead to the production of muscular tissue ? 

 Contraction so invariably follows upon stimulation, that 

 ■we may safely say the cause in question can be no other 

 than the incidence of certain stimulations — and we pro- 

 bably shall not be very far wrong if we suppose that these 

 result from, or take their origin in, shocks or other physical 

 impressions upon definite though related parts of the 

 external surface of the organism. Its form or its mode 

 of progression by cilia may lead it to come into contact 

 with external objects most frequently by some particular 

 part of its surface, and such local shocks produce waves of 

 molecular movement, which pass more especially in some 

 one or more directions and act as stimuli. 



It is pretty certain that impressions or shocks made 

 upon protoplasm, or even the incidence of physical agents 

 such as light or heat, liberate molecular movements 

 therein, and that these molecular movements may be 

 transmitted from their point of origin through it in all 

 directions. Yet it occasionally happens, owing to the 



