CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE 

 MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE. 

 E. L. MARK, DIRECTOR. — No. 13G. 



MENDEL'S LAW OF HEREDITY. 

 Bv W. E. Castle. 



Presented January 14, 1903. Received December 20, 1902. 



What will doubtless rauk as one of the great discoveries in biology, 

 and in the study of heredity perhaps the greatest, was made by Gregor 

 Mendel, an Austrian monk, in the garden of his cloister, some forty years 

 ago. The discovery was announced in the proceedings of a fairly well- 

 known scientific society, but seems to have attracted little attention and 

 to have been soon forgotten. The Darwinian theory then occupied the 

 centre of the scientific stage and Mendel's brilliant discovery was all but 

 unnoticed for a third of a century. Meanwhile the discussion aroused by 

 Weismann's germ-plasm theory, in particular the idea of the non-inheri- 

 tance of acquired characters, had put the scientific public into a more 

 receptive frame of mind. Mendel's law was rediscovered independently 

 by three different botanists engaged in the study of plant-hybrids, — de 

 Vries, Correns, and Tschermak, — in the year 1900. It remained, how- 

 ever, for Bateson, two years later, to point out the full importance and 

 the wide applicability of the law. This he has done in two recent pub- 

 lications with an enthusiasm which can hardly fail to prove contagious. 

 There is little danger, I think, of Mendel's discovery being again forgotten. 



1. The law of dominance. When mating occurs between two animals 

 or plants differing in some character, the offspring frequently all exhibit 

 tlie character of one parent only, in which case that character is said to 

 be ^^ dominant^ Thus, when white mice are crossed with gra}' mice, all 

 the offspring are gray, that color character being dominant. The char- 

 acter which is not seen in the immediate offspring is called " recessice" 

 for though unseen it is still present in the young, as we shall see. Wliite, 

 in the instance given, is the recessive character. The principle of 

 heredity just stated may be called t/ie law of dominance. The first in- 

 stance of it discovered by Mendel, related to the cotyledon-color in peas 



