4 GENETIC VARIATIONS 



There occur great numbers of variations that affect only 

 individuals, having no influence on their descendants. Color 

 becomes changed under action of the sun, but this does not 

 affect the offspring of the individual concerned; and there 

 are thousands of such variations. 



Thus for a variation that plays a role in evolution — for a 

 genetic variation — two things are essential. There must be a 

 change. And there must be inheritance of the changed con- 

 dition. A non-genetic variation is like a change in the shape 

 of a crystal, resulting from filing off its angles and edges. Such 

 a crystal, when dissolved and recrystallized will produce 

 anew the original form, for its chemical constitution has not 

 been changed by the alteration in its shape. A genetic varia- 

 tion is like an alteration in the chemical constitution of the 

 crystal, causing it when dissolved and recrystallized, to appear 

 in a new form; a new form that is repeated as often as 

 crystallization occurs without further change in chemical 

 constitution. 



Certain other relations play a great role in the results of 

 genetic variation. Variations occurring in certain parts of 

 organisms cannot be inherited. To influence later generations 

 they must affect certain particular parts of the body or of the 

 cell. In all higher organisms, reproduction does not occur by 

 mere division of the body of the parent into two equal parts, 

 as may occur in bacteria and protozoa. On the contrary, the 

 new individuals arise from single cells, "germ cells," which 

 are given off by the parents. These germ cells arise from 

 certain well defined parts of the body, the gonads or germ 

 glands. Thus any genetic variations that are to be carried on 

 to later generations must affect the germ cells. If a genetic 

 variation arises in some other part of the body — say in the 

 hand — this cannot be carried on into later generations, for 

 no part of the later generation arises from the hand. Whether 



