350 MENDEL'S LAWS 



In many cases the variation might be known to be due to 

 soil or climate or some other environmental factor. In 

 any event the variations would not be transmitted to 

 their ofifspring. This type of variation is called modifica- 

 tion. 



Mutations. — Even in the purest races or in pure lines 

 variations sometimes occur which are transmitted to suc- 

 ceeding generations. They are known as mutations. A 

 not uncommon illustration of mutation is the appearance 

 of a white-flowered plant in a pure line with colored 

 flowers. If self-pollinated such white plants almost in- 

 variably breed true generation after generation irrespec- 

 tive of the climate or conditions of cultivation. 



Recombinations. — When parents differ in several 

 characters the hybrid offspring usually exhibit some of the 

 characters of each parent and also lack some from each 

 one. Variations bf this sort due to recombination of char- 

 acters are neither modifications nor mutations. 



Heredity is the process which is responsible for the 

 particular combination of transmissible characters pos- 

 sessed by any organism. It should be noted that the 

 definition makes heredity responsible for the differences 

 between parents and offspring as well as for their re- 

 semblances. Probably an illustration will make this clear. 

 When a pure brown-eyed man marries a pure blue-eyed 

 woman the children all have brown eyes but if these 

 brown-eyed offspring marry blue-eyed mates approxi- 

 mately half their children will have blue eyes and half 

 brown eyes. The process of heredity is equally responsi- 

 ble for both results. 



Mendel's Discovery of the Laws of Heredity. — 

 Although the fact of heredity has been known for two 

 thousand years or more the laws have only recently been 

 discovered. The first two of them we owe to an Austrian 

 monk, Gregor Mendel, who experimented with garden 

 peas and published an account of his work in 1865, in 

 which he made the first statement of what we know today 



