288 EVOLUTION AND GENETICS 



individuals will breed true. One thing alone cannot be stabilized 

 as an independent strain through ordinary methods of reproduc- 

 tion, and that is a variety based on the heterozygous condition, 

 such as the blue Andalusian fowl. These must be maintained })y 

 repeated crossing or by breeding heterozygous individuals and 

 selecting their heterozygous offspring. 



Summary. Mendel established the fact that organisms are 

 made up of unit characters which are segregated during repro- 

 duction. These characters may be alternative, blending or mosaic 

 in the hybrid, but in the offspring of the hybrid they are reassoci- 

 ated in definite ratios, so that the characters of the original 

 parents as well as of the first hybrid reappear. The ratios are 

 fixed according to the law of chance for every number of characters. 

 They are only approximated in breeding experiments unless large 

 numbers of individuals are considered. Many organisms are now 

 used for experimental studies in this field, but fruit flies of the 

 genus Drosophila are the most important. The laws are of great 

 practical importance to breeders of plants and animals. 



REFERENCES 



Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B., 



The Mechanisyn of Mendelian Heredity, revised edition, 1922. 

 Morgan, T. H., The Physical Basis of Heredity, 1919. 



All general works on heredity contain more or less detailed accounts of Men- 

 delian inheritance. Those cited above are especially valuable for detailed 

 evidence of the various processes as illustrated in Drosophila. 



