The Source of Variability 95 



be made in a complex biological compound without destroying it. 

 However, the primary influence regarding genetic change is physio- 

 logical. A change must act harmoniously in relation to the rest of 

 the genome with respect to initiating and controlling the many 

 chemical pathways involved in duplicating the organism. If the 

 organism or colony bearing the change is not duplicated, the change 

 is not perpetuated. Lethal mutants represent such changes. Thus 

 the fact that a change is possible chemically does not necessarily 

 mean that it will produce a balanced genetic system. Changes in 

 chromosome structure have the added physical prerequisite that 

 they must produce a mechanical assemblage capable of synapsis 

 and division at meiosis. Many chromosomal and genie mutations do 

 occur that are not perpetuated because of these several physio- 

 logical processes. The genetic changes which are effectively in- 

 troduced into the living population are thus restricted by the 

 physiological framework into which they must fit. 



