LECTURE V. 



U N I T Y I N L I F E. 



Essential unity amid variety. Persistence of plan. 

 Types of structure very few. Embryology shows like 

 plan in organisms seemingly unlike. All organisms 

 are cells or clusters of cells filled with protoplasm ; 

 each cell provided with a nucleus and its chromatin. 

 and with the associated structures hy which the work 

 of a cell is performed. 



Homology. Its existence; its significance; its dis- 

 tinction from analogy. Homology in structure; ho- 

 mology in development ; man and dog ; man and 

 alligator. Homology through descent. Homology 

 through common descent. Common descent the sole 

 known source of homology. Homology means blood 

 relationship. Homology is the stamp of heredity. 

 The law of unity has its basis in the influence of 

 heredity. 



All laws of life apply to man, to the low T er animals, 

 and to "our brother organisms, the plants," each in 

 its degree. 



Principal factors in Organic Evolution: 



/. Heredity. " Like begets like " ; creatures resem- 

 ble their ancestors. 



' //. Irritability. All living beings respond to ex- 

 ternal stimulus. They are moved by or they react 

 from every phase of their surroundings. 



