CHAPTER XII 



TYPE AND VARIABILITY 1 



Enough has been shown in earlier chapters to convince the 

 student that variability is an inevitable accompaniment of both 

 reproduction and development, and therefore that variation is to 

 be expected among living beings everywhere. 



Before anything like a comprehensive idea of transmission 

 can be developed therefore, it is necessary to study, not indi- 

 viduals, but groups, and to establish definite conceptions as to 

 type and variability. In this, as in any other critical study of a 

 race, we proceed character by character, and are careful to 

 include enough individuals to be fairly representative of the race 

 as a whole. 2 



A farmer plants an ear of corn say ten inches in length. 

 What he gets is not a crop of ears all ten inches long, but a 

 group of ears ranging in length all the way from perhaps three 

 or four inches up to eleven or twelve, or even a little more. The 

 same principle will hold if the ear that is planted is nine inches 

 long instead of ten, except that the distribution will be different, 

 lengths running, in general, slightly lower; that is to say, the 

 length of ear of the offspring is not the same as that of the 

 parent, but it constitutes a "distribution " extending both above 

 and below that length. So far as is known, this principle of trans- 

 mission holds true in all races and for all characters. Stated in 

 more general terms, we may say that the offspring as a whole is 

 not the same as the immediate parents, but that it constitutes a 

 distribution extending from near the lower to approximately the 

 upper limits of the race. This suggests at once the idea of type 

 and that deviation from type which ^ve call variability. 



1 See Bulletin No. 7/9, Agricultural Experiment Station, University of Illinois, 

 by the author of this text. 



2 Having discovered the type as to several important characters, it would then 

 be possible to select a typical individual. 



419 



