722 General and Applied Biology 



conclusion that a particular character is being inherited when in reality 

 it is merely a natural, normal variation. By taking advantage of varia- 

 tions, improvements in races of organisms can be made. New varieties are 

 often the result of the proper selection and development of variations. 

 A study of variations reveals that all life is constantly changing. As 

 often stated, the most invariable thing in nature is variability. The suc- 

 cessful plant or animal is the one which successfully can take advantage 

 of its numerous variations. 



CLASSIFICATION OF VARIATIONS 



According to Their Heritability. — Somatic modifications or acquired 

 characters are not thought to be inheritable, except in a few instances. 

 Somatic modifications are structural and functional adjustments of the 

 bodies (soma) of individuals to differences in environment, either dur- 

 ing or after their embryologic development. Examples of somatic modi- 

 fications are muscular changes, calloused hands, tanned skins, acquired 

 information, and acquired injuries to animals and plants. 



Combinations result from the combining of the hereditary characters 

 of two different races or strains, with the production of nothing distinctly 

 new but a mere combining of the old characters. Certain types of com- 

 binations are inheritable. For instance, the angora coat of a mother 

 guinea pig might be combined with the black color of the father to pro- 

 duce a combination of angora and black in the offspring. 



Mutations or hereditary variations are always inherently inheritable 

 if they are to be classed as mutations. They are variations resulting from 

 spontaneous chemical changes in the chromatin of the cells or an ab- 

 normal number of chromosomes or their contained genes. (For a more 

 detailed discussion of mutations, see the chapter on Heredity.) An 

 example of a mutation is the production of a fruit fly with a narrow, 

 elongated eye (called bar eye) from a normal, oval-eyed form. 



According to Their Nature. — Morphologic or structural variations 

 (Fig. 357) are usually differences in size, shape, color, or number of 

 structures. 



Physiologic or functional variations are usually differences in such 

 things as vitality, nutrition, productivity, or secretions. These variations 

 are usually a necessary corollary of morphologic variation. 



Psychologic variations are variable expressions of the nervous system 

 or the inherent sensitivity of living protoplasm. Such things as responses, 

 dispositions, and mental abilities are well-known variations in this field. 



