90 THEORY OF EVOLUTION 



few familiar, historical facts, that preceded the 

 discovery of the mechanism in qnestion. 



Throughout the greater jjart of the last cen- 

 tury, while students of evolution and of hered- 



Viv,. -1-.5. Typical Cfll sliowiii^' the cell wall, the protoj)lasm 

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

 chrou)atin and nuclear sap. (After Dahlgren.) 



ity were engaged in what I may call the more 

 general, or, shall I say, the grofiser asj^ects of 

 the suhject, there existed another group of stu- 

 dents who were engaged in working out the 

 minute structure of the material hasis of the 

 living o]-ganism. They found tliat organs such 

 as the hrain, the heart, the liver, the lungs, the 

 kidneys, etc., are not themselves the units of 

 structure, hut that all these organs can he re- 

 duced to a simpler unit tliat repeats itself a 



