3IO READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



four classes: morphological, physiological, psychological and eco- 

 logical. 



Morphological variations are differences in size and form. In 

 general morphological variations have more significance for the 

 biologist than for the agriculturist. However, in many products of 

 the farm, size and conformation are of decided importance. Two 

 sub-classes under morphological variations are merisiic and homeotic 

 variations. Meristic variations are differences in number of repeated 

 parts such as the petals in a flower, the leaflets in a compound leaf or 

 number of phalanges. Homeotic variations are differences caused by 

 the replacement of one part by another, as the production of an 

 antenna in place of an eye in an insect. 



Physiological variations are differences in quality and performance. 

 Examples of qualitative variations are difference in degree of hardness 

 of bone, flavor of meat, richness of milk, difference in normal color, 

 resistance to drouth, frost or alkah. Variations in performance con- 

 stitute the most important group for the producer. Differences in 

 performance are sometimes, though not necessarily, associated with 

 certain details of structure. 



Psychological variations are differences in mental traits. That 

 mental and nervous conditions have very definite effects upon physical 

 conditions is well known, but the problem of distinguishing between 

 purposeful action and automatic response, between manifestations of 

 reason and manifestations of instinct, is set for the students of animal 

 beha\'ior. WTaile variations in mental characteristics have an impor- 

 tant place in eugenics and merit the attention of livestock breeders, 

 yet the inheritance of psychological characters must be more exten- 

 sively 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 

 variation in crop yield and hence is of immediate sigm'ficance to 

 agriculture. 



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 



