90 THEORY OF EVOLUTION 



few familiar, historical facts, that preceded the 

 discovery of the mechanism in question. 



Throughout the greater part of the last cen- 

 tury, while students of evolution and of hered- 



FIG. 45. Typical cell showing the cell wall, the protoplasm 

 (with its contained materials) ; the nucleus with its contained 

 chromatin and nuclear sap. (After Dahlgren.) 



ity were engaged in what I may call the more 

 general, or, shall I say, the grosser aspects of 

 the subject, there existed another group of stu- 

 dents who were engaged in working out the 

 minute structure of the material basis of the 

 living organism. They found that organs such 

 as the brain, the heart, the liver, the lungs, the 

 kidneys, etc., are not themselves the units of 

 structure, but that all these organs can be re- 

 duced to a simpler unit that repeats itself a 



