STRUCTURE OF THE GERM PLASM 9 



that known for the genes. C. Much enlarged chromosomal threads of 

 the lily, showing the chromomeres (genes?), after Belling, 1928. 

 D. The chromosomes in the condensed condition (chromomeres not 

 visible), in the female fruit fly, Drosophila melanogaster, after 

 Metz, 1916. 



Structural or physiological, may be changed by altering one or 

 more of the genes ; by substituting one gene for another. The 

 entire practice of genetic science, of animal breeding, is an 

 elaborate technique for substituting one gene for another and 

 determining the consequences of such substitution. 



Different individuals are found to have genes that differ 

 from each other in some of their properties; in consequence 

 of this, different individuals have different characteristics. To 

 this fact are due most of the diversities between individuals; 

 differences in features, in body form, in size and in struc- 

 ture; differences in physiology; differences in senses and 

 behavior; differences in character and personality. With in- 

 crease of knovi^ledge it has become more and more clear that 

 most differences of all kinds betw^een individuals are due to 

 differences in the materials of w^hich they are made; that is, in 

 the genes that they carry — rather than to differences in the 

 conditions under v^^hich they have lived. 



When a new individual is formed— constituting at the 

 beginning a new fertilized tgg — this new individual gets half 

 his genes from his mother, half from his father. And now a 

 most important fact! The genes that he gets from one parent 

 — as from the mother — form a complete series, containing 

 genes for every function in development. This series contains 

 all the genes that are necessary for development and for pro- 

 ducing all the structures and functions of the body. 



And the other chain of genes, that he gets from his other 

 parent, likewise contains all the genes that are necessary for 



