22 GENETICS 



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. 



f. 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 

 variations may form a continuous series, grading into 

 each other by intermediate steps, or they may be dis- 

 continuous 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 hun- 

 dred to the lowest. 



On the other hand the number of segments in the 

 abdomen of a shrimp, for instance, which may be either 

 eight or nine but cannot be halfway between, illustrates 

 what is meant by discontinuous variation. The wide- 

 spread occurrence of this later category of variations 

 has been pointed out by Bateson in his encyclopedic 

 volume "On Materials for the Study of Variation." 



h. With respect to their character variations may be 

 quantitative or qualitative. A six-rayed starfish rep- 

 resents a quantitative variation from the normal num- 

 ber of five rays, whereas a red variety of a flower may 

 differ chemically from a blue variety, or a bitter fruit 



