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 
Fic. 2.—Substantive variation due to chlorophyll reduction in certain areas of the leaves 
of Eleagnus 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 beobserved 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. 
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