VARIATION 31r 
continuous variations and the second, abnormal, definite and discon- 
tinuous variations. It should be noted, however, that all discontin- 
uous variations are not necessarily definite or even distinguishable. 
Continuous variations when examined statistically are found to con- 
form to the law of statistical regularity. That is, if measured and 
plotted the graph will approximate the normal curve of variability. 
Continuous variations are either heritable (combinations) or non- 
heritable (modifications) and, as was stated above, the only certain 
method of determining the class in which a given case may fall is the 
breeding test. Discontinuous variations are essentially discrete dif- 
ferences whether they be large or small. They are also either herit- 
able and there is no correlation between size and heritability. Thus 
the extremely large and small mustard plants, considered by them- 
selves, are discontinuous variations, but they are almost certainly due 
entirely to environmental differences and seed fr m 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 
individuals 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. Occa- 
sional examples are found among short-lived or vegetatively propa- 
gated species. The remarkable series of variations of the Boston 
fern is a good example. Fortuitous 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 be- 
tween germ and soma. : 
Variation and development.—Somatogenesis, in sexually produced 
multicellular organisms, includes the entire history of cellular multi- 
plication 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 gameto- 
genesis, 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 
