HEREDITY AND VARIATION 211 



tions merely affect the individual and do not reappear in the 

 offspring unless the particular environment which has caused 

 them persists. Acquired variations of this kind arc of particu- 

 lar interest to the farmer or to anyone who is concerned with 

 plant culture, since they can readily be controlled by a proper 

 manipulation of the environment ; but the breeder and the student 

 of inheritance must learn to recognize them and to realize that 

 they are quite valueless for his purposes. Good care and cultiva- 

 tion will bring out the best that there is in a poor race of plants 

 but they can never change a poor race into a good one. 



Mendel's Law of Inheritance.— It is therefore only with those 

 characters which are clearly inherited that we are here concerned. 

 As we study their behaviour in the passage from generation to 

 generation we notice many apparent irregularities. The same 

 parents will transmit different characters to their different 

 offspring. Sometimes a particular parental character will fail 

 to appear in the offspring at all. In other cases the offspring 

 will develop characters possessed by neither of its parents but 

 sometimes found in a more remote ancestor. It is these facts 

 which geneticists are endeavoring to explain and to reduce to 

 definite laws. The most notable of these laws, and the one which 

 is the basis of our modern understanding of inheritance, is that 

 formulated by Gregor Mendel (Fig. 122), an Austrian monk, in 

 1866. The great importance of his work was ignored at the time 

 and was not recognized until 1900. Since that date a large 

 number of investigators have diligently studied the applications 

 of "Mendelism" in the inheritance of a wide variety of animals 

 and plants, and although our advancing knowledge has resulted 

 in many amplifications and interpretations of this law, it still 

 remains fundamentally intact as the cornerstone of genetics. 



In his cloister garden Mendel studied inheritance in peas, 

 making hj^brids between different types and studying the results 

 from generation to generation. His method of attack on the 

 problem differed in several important ways from that of previous 

 workers. First, in his crosses between contrasting types Mendel 

 would single out a particular character of the plant and follow 

 out its behaviour by itself, rather than trying to study the whole 

 complex individual at once. In this way the inheritance of 

 flower color, of seed color, of seed surface, of plant height, and of 

 several other characteristics of the garden pea were investigated. 

 Second, he kept accurate pedigree records, making sure that he 



