VARIATION 227 



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 distiaguishable. 

 Continuous variations when examined statistically are foand 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 or non-heritable, and there is no correlation between size and 

 heritability. Thus the extremely large and small mustard plants, con- 

 sidered by themselves, are discontinuous variations, but they are 

 almost certainly due entirely to environmental differences and seed 

 from the small plant if grown under optimum conditions would pro- 

 duce 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 =0 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 his activity depends upon 



