214 THE PERPETUATION OF ADAPTATIONS 



of the female parent, these characteristics will be transmitted 

 by the spermatozoa E, E and not by spermatozoa E'E', which 

 transmit the male parent equivalent of these characteristics. 

 After fertilization with these spermatozoa the offspring of E 

 will inherit this set of characteristics from the paternal grand- 

 mother, while those from E' will inherit from the paternal grand- 

 father, subject of course, in both cases, to modifications brought 

 into the union by the egg cells in which similar maturation 

 processes have taken place. 



E. THE MENDELIAN PRINCIPLES OF HEREDITY 



The preceding account of the cytological changes during 

 maturation and their interpretation on the basis of Weismann's 

 theory of the germ plasm indicates, if the theory is correct, that 

 the germ cells when ready for fertilization are pure in respect 

 to any given characteristic, i.e., they carry inheritance of that 

 characteristic from either the male or the female parent and 

 not from both. 



The same conclusion was reached entirely independently of 

 cytology or of Weismann's hypotheses through experimental 

 breeding of plants and animals, and is embodied in the so-called 

 Mendelian principles of heredity. The great field of modern 

 genetics is the outcome of experiments in selected breeding 

 first carried out scientifically in 1865 by Gregor Mendel a 

 Silesian monk in the gardens of the monastery at Briinn. The 

 results of his experiments and the conclusions he drew from them 

 were published in an obscure journal where they remained 

 buried for thirty-five years. In the year 1900 the botanists 

 de Vries, Correns and Tschermak working independently each 

 brought out evidence confirming Mendel's conclusions and the 

 full value of his work was finally recognized. Soon after, 

 Bateson demonstrated that Mendel's principles apply to 

 animals as well as plants. 



A. HEREDITY OF ONE PAIR OF CHARACTERS. Mendel's 

 principles of heredity can be best illustrated by a simple case 

 of his own involving only one pair of characters in which one 



