VARIATION 311 



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 difi'erences 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 arnong short-lived or vegetatively propa- 

 gated species. The remarkable series of variations of the Boston 

 fern is a good example. Fortuitous variations are chance ditTerences 

 occurring in all directions. 



5. According to cause variations are either ectogcnctic, differences 

 arising from conditions acting upon the organism from without; or 

 autogenetic, differences resulting from strictly internal relations be- 

 tween germ and s^ma. 



Variation and development. — Somatogenesis, in sexually produced 

 multicellular organisms, includes the entire history o' cellular mulli- 

 phcation and speciaHzation 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 gamcto- 

 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 



