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. 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 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 



