16 GENETICS IN RELATION TO AGRICULTURE 
comprise a large portion of what are commonly spoken of as fluctuations 
due to environment, but all cases of fluctuating variation are not modifica- 
tions inasmuch as variations due to combinations frequently display the 
normal variability curve also. Modifications are not heritable. The 
second category, variation by combination of hereditary units is often 
confused with modification, as already stated, because of the fact that 
variations caused by segregation and recombination when studied statis- 
tically often display the normal variability curve. This is especially 
apt to be the case in quantitative characters (those of size or weight) 
and segregation and recombination may be the cause of gradations in 
color intensity. In autogamous (self-fertilized) organisms hybridization 
between races is sufficiently rare to be negligible ‘n this connection, 7.e., 
in such species the fluctuating variations are caused by the environment. 
But in allogamous organisms (those in which two individuals are neces- 
sary to accomplish sexual reproduction) fluctuating variations may be 
caused either by the environment, by segregation and recombination of 
factors, or by both causes acting together. We shall take up the third 
category, mutations, in a later chapter. For the present it is sufficient 
to remember that mutations are no doubt the least frequent of the three 
classes, that easily distinguishable mutations are comparatively rare, 
but that there may also occur true mutations of such moderate extent, 
as compared with the population, that their existence would only be 
detected by breeding tests, since their progeny would exhibit a different 
range of fluctuation from that of the population. 
2. Nature. We may next enquire into the nature of variation as 
it affects the organism. Upon this basis we may distinguish between 
four classes: morphological, physiological, psychological and ecological. 
Morphological variations are differences in size and form (Fig. 1). 
In general morphological variations have more significance for the biolo- 
gist 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 replace- 
ment 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 
(Fig. 2), resistance to drouth, frost or alkali. Variations in performance 
constitute the most important group for the producer. Differences in 
performance are sometimes, though not necessarily, associated with 
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