250 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



Stentor this substance presents the appearance of 

 longitudinal threads ; in Gregarina they encircle the 

 body. From these simple beginnings we can follow 

 the muscular tissue to its various expressions in all 

 classes of animals ; to the concentric threads of the 

 Coslenterata ; to the longitudinal bundles beneath the 

 integument of worms ; and the variously directed 

 masses attached to the external skeleton of the Arthro- 

 poda, and the internal skeleton of the Vertebrata. The 

 ease with which muscular tissue is grown in the higher 

 animals under use, permits us to infer that its develop- 

 ment in all animals has been due to the same cause. 

 Muscular structure is directly related to the needs of 

 the structures to which it is attached, in the perform- 

 ance of movements. In rudimental limbs muscles are 

 reduced in both size and number, distinct slips or 

 bodies becoming fused. In enlarged limbs the reverse 

 process takes place. Muscular bellies increase in size, 

 and in number by subdivision. The muscular system 

 in the middle and higher Vertebrata is variable, and its 

 plasticity to the stimuli to movements is well known. 

 It is evident that definite muscular bands have been 

 developed in the lines of resistance which it has been 

 necessary to overcome in moving the body or parts of 

 it. The movable segments which have become adapted 

 for contact with the surrounding media, by development 

 of hardness or extent of surface, as limbs (feet, wings), 

 and jaws, have naturally become the points of origin 

 of the most efficient muscular bands. No one can 

 doubt the mutual stimuli which the muscular and 

 skeletal systems have exchanged during the process of 

 evolution, since they are necessary to each other from 

 a mechanical point of view. 



