20 GENETICS IN RELATION TO AGRICULTURE 
mustard plants shown in Fig. 3 considered by themselves are discontinu- 
ous variations, but they are almost certainly due entirely to environ- 
mental differences and seed from the small plant if grown under optimum 
conditions would produce plants of normal size. On the other hand, it 
is known that many minute differences in organisms are heritable. 
4. According to direction variations are classed as orthogenetic and 
fortuitous. Orthogenetic variations are those differences found in indi- 
viduals related by descent which form progressive series tending in a 
definite direction. Many remarkable illustrations are found among 
paleontological records of the evolution of animals. Occasional examples 
are found among short-lived or vegetatively propagated species. The 
remarkable series of variations of the Boston fern described in Chapter 
XVI is a good example. Fortuitows variations are chance differences 
occurring in all directions. 
5. According to cause variations are either ectogenetic, differences 
arising from conditions acting upon the organism from without; or 
autogenetic, differences resulting from strictly internal relations between 
germ and soma. 
Variation and Development.—Somatogenesis, in sexually produced 
multicellular organisms, includes the entire history of cellular multipli- 
cation and specialization from the first cleavage of the fertilized (or 
parthenogenetic) egg to the completion of all adult features. From the 
standpoint of individual development it includes gametogenesis, for the 
production of sexual glands and of secondary sexual characters are merely 
phases of differentiation. Cell growth and cell function depend directly 
upon the activity of the living substance within the cell. The nature 
and degree of this activity depends upon two sets of determining causes 
acting simultaneously. First, there are the specific hereditary determiners 
or genetic factors, which react with the other elements of the protoplasm 
and, under favorable circumstances, condition normal development. 
Second, there are all the conditions external to the cell which stimulate or 
inhibit protoplasmic activity. These “developmental stimuli” are chem- 
ical and physical changes wrought by energy from without the organism or 
caused by its own physiological activities. Chemical stimuli are exerted 
mainly through the medium of the circulating liquid which surrounds 
each living cell. Normally this fluid contains the elements essential for 
maintenance of life as well as various waste products. It may also bear 
toxic substances that suppress or inhibit the cell functions and in higher 
animals it contains the secretions of the ductless, sexual and other glands 
that profoundly affect development. Physical stimuli are exerted 
chiefly from without and upon the organism as a whole. They include 
changes in temperature, light and density of medium, the effects of 
electric and radiant energy, force of gravity, etc. Obviously, so many 
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