254 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



frequent repetition of these local contractions and en- 

 largement of the fiber have resulted in a permanent 

 difference in its intimate structure, the alternate waves 

 becoming fixed as cross-bands." 



As evidence of the truth of this proposition, (6), 

 Eimer cites many facts. In the muscles of the Mol- 

 lusca, striped fibers occur in those forms, as Pecten, 

 where the closing of the shell is especially vigorous, 

 this being their mode of progress through the water. 

 In other forms, where the muscles have no such vigo- 

 rous use, the fibers are smooth. In Arthropoda, the 

 muscles of the legs of swiftly running forms are striped, 

 while those of the alimentary canal are smooth. It is 

 a general law that muscles which have energetic con- 

 tractions are striped, while those in which the con- 

 tractions are slow or feeble, are smooth. In the com- 

 mon house-fly, Eimer records some remarkable obser- 

 vations. Flies examined in winter, during the period 

 of torpidity, were found to have the fibers of the tho- 

 racic muscles smooth. With advance of the spring 

 the striping gradually made its appearance, and in the 

 summer it was fully developed. An artificial imitation 

 of winter, by refrigeration in an ice-cellar, caused the 

 cross-striping to disappear. The striping in some 

 other animals is shown by Eimer to be strongly in- 

 fluenced by physical conditions. 



In fact, muscular tissue is highly plastic, and as it 

 is directly under the control of nervous or equivalent 

 stimuli, the effect of the latter in building structure is 

 evident. That the motion communicated to the hard 

 parts through the agency of the muscular system is 

 effective in building the hard structures will be shown 

 in a subsequent section. 



