18 



GENETICS IN RELATION TO AGRICULTURE 



of pyschological characters must be more extensively investigated before 

 the subject can be considered with profit in a fundamental study of 

 genetics. 



Ecological variations are those differences between individuals that 

 result from their fixed relation to the environment. These differences 

 are especially noticeable in plants and are known as place-effects or 

 place variations. This category includes some of the phenomena of 



Fio. 2. Substantive variation due to chlorophyll reduction in certain areas of the leaves 



of Elceagnus pungens. 



variation in crop yield and hence is of immediate significance to agricul- 

 ture. Fig. 3 illustrates place-effects in a common weed. 



3. According to differences between them there are two general classes 

 of variations: first, the slight differences in every character which are 

 always to be observed even among individuals of identical heredity; second, 

 unusual, striking differences commonly known as sports. The first class 

 are called normal, indefinite fluctuating or continuous variations and the 

 second, abnormal, definite and discontinuous variations. It should be 

 noted, however, that all discontinuous variations are not necessarily 

 definite or even distinguishable. Continuous variations when examined 

 statistically are found to conform to the law of statistical regularity. 



