40 GENETICS 



cessful line stands out with distinctness, makes it easy 

 for him to view successive variations as orthogenetic, 

 that is, as definitely directed in one course either 

 through intrinsic (Nageli) or extrinsic (Eimer) causes. 



Fortuitous or chance variations in all possible 

 directions furnish the repertory of opportunity, 

 according to Darwin, from which natural selection 

 picks out those best adapted to survive in the strug- 

 gle for existence. 



e. With respect to their source, variations may be 

 somatic or germinal. Somatic, or body variations, 

 arise as modifications due to environmental factors. 

 They are individual differences which may be quite 

 transitory in nature, while germinal variations may 

 arise without regard to the environment, are deep- 

 seated, and of racial rather than of individual sig- 

 nificance. 



/. With respect to their normality variations may 

 fall within expected extremes and thus be considered 

 normal, or they may be outside of reasonable expec- 

 tations and consequently be reckoned as abnormal, 

 as in the case of a two-headed calf. 



g. With respect to the degree of their continuity varia- 

 tions may form a continuous series, grading into each 

 other by intermediate steps, or they may be discon- 

 tinuous in character. An example of continuous 

 variation is the height of any hundred men one might 

 chance to meet, which would probably represent all 

 intermediate grades from the highest among the 

 hundred to the lowest. 



The number of segments in the abdomen of a 



