310 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 meristic 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 alkali. 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 
behavior. While 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 significance 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 
