INVESTIGATIONS SINCE MENDEL 547 



lication and sixteen years after Mendel's death. In 1900 his 

 paper was discovered simultaneously by three students of 

 genetics, Correns, De Vries, and Tschermak, who recognized 

 its importance. Since that time Mendel's law has formed the 

 basis of all work in genetics. 



The Value of Mendel's Discoveries. — Mendel's discoveries 

 have completely revised our methods of investigating and ideas 

 concerning heredity. First, Mendel's discoveries have im- 

 pressed upon us the value of pedigree cultures in investigating 

 problems of heredity. Second, they afford us laws concerning 

 the appearance of characters in the offspring, whereby we know 

 what to expect and can thereby interpret results which were 

 previously a medley and not understandable. Third, knowing 

 how characters behave in the offspring when plants or animals 

 are crossed, we can start in our crossing work with definite 

 results to be obtained in mind and also plan a definite method 

 of procedure to obtain the desired results. Fourth, owing to 

 the discovery of the segregation of characters, we now know 

 that, in the second generation of hybrids, individuals that are 

 perfectly pure occur in definite proportions and that purity of 

 plants and animals in respect to a character does not depend 

 upon a long series of selections as was formerly the notion. 

 Fifth, the law of dominance explains why plants or animals 

 impure in respect to a character may appear just as pure as 

 pure individuals. Sixth, knowing that some characters are 

 recessive and are entirely obscured by the contrasting domi- 

 nant characters, we can now explain the appearance in the 

 offspring of a character which did not appear in the parents or 

 even for generations back, and in this way account for many of 

 the variations in offspring, such as talented offspring of medi- 

 ocre parents, blue-eyed children of brown-eyed parents, bad 

 sons of pious preachers, rust-resistant plants of plants suscep- 

 tible to rust, and so on. Seventh, Mendel's discovery that pairs 

 of contrasting characters behave independently of each other 

 but may be combined in various ways makes it possible for us to 

 improve plants and animals by breeding them in such a way as 

 to bring the desirable characters of different varieties together 

 in one individual. 



Investigations since Mendel. — Since the discovery of Men- 

 del's paper, numerous investigators have been applying and 



