64 The Unity of the Organism 



the muscle fibers. So it would be merely a matter of suf- 

 ficient patience to go over all the tissue systems, epithelial, 

 glandular, bony, nervous, and the rest, and point out nu- 

 merous certain, and innumerable probable instances of dif- 

 ferences for different taxonomic groups of animals, and to 

 show that these hereditary differences are expressed pri- 

 marily in the cytoplasm of the cells. 



Summary of Positive Information about the Physical Basis 



of Heredity 



We have explored a vast region of fact and theory con- 

 cerning propagation and development in organisms, for the 

 purpose of ascertaining what is actually known about the 

 organs and substances by which hereditary attributes are 

 produced. Expressing the matter in terminology familiar 

 to current discussion on heredity, we have been trying to 

 find what is actually known about the physical basis of 

 heredity. If clear-cut, unequivocal information of the 

 kind sought is contained in all we have seen, it ought to be 

 statable in a few simple sentences. 



What has been accomplished may be epitomized in two 

 such sentences: 



First. Overwhelming observational evidence has been se- 

 cured that the cytoplasm of cells participates directly in 

 the formation of organic parts which have hereditary at- 

 tributes. 



Second. A great mass of evidence, partly of observation 

 and partly of legitimate inference from the principles of 

 organic integration, has been secured, that the chromosomes 

 of the germ-cells in plants and animals which propagate by 

 means of such cells, participate in the production of or- 

 ganic parts having hereditary attributes. 



Any substance which plays such parts in development 

 may be named a physical basis of heredity; and these two 



