HEREDITY. 



337 



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GB 



GB' 



GB 



GB 



foundly dissimilar from the point of vie^v of their hereditary vahie. 

 the residts are necessarily erroneous. 



Finally, however, the application of the experimental method at 

 once revealed precise laws which threw a bright light upon the phe- 

 nomena of heredity. It is necessary that I should detail with some 

 care the fundamental experiment figured in the diagram (fig. 1). 



You are acquainted with the wild gray mouse on the one hand 

 and the white or rather albino mouse, which can be readily procured 

 of dealers in animals, on the other ; these two forms differ sharply from 

 each other ; the gray mouse has colored hair and black eyes ; the white 

 mouse has pure Avhite hair and pink eyes, that is to say it is lacking 

 in coloring matter. AVe may take these two forms for the purpose of 

 investigating the laws of heredity as 

 regards these tAvo complementary 

 characters, that is, as regards the 

 presence or absence of color. 



The crossing between these two 

 forms constantly furnishes offspring 

 absolutely like the gray parent with 

 black eyes; there is no mixture be- 

 tween the two complementary char- 

 acters, no mixed form ; it is the gray 

 parent exclusively that reappears in 

 the progeny ; it is then said that there 

 is a dominance of the gray character ; 

 the white character which is not ex- 

 pressed, which is hidden by the other, 

 is said to be dominated or latent. 



But let us continue the experiment ; 

 let us cross with each other the hy- 

 brids having the dominant gray char- 

 acter. There appears this time in the progeny some gray ones 

 still, but there are also white ones with red eyes, the latter being 

 less in number than the former. If a considerable number of cross- 

 ings has been made so as to have some hundreds of specimens, 

 it will be seen that there is always a fixed numerical relation between 

 the two kinds, a relation which is that of three grays for every ojie 

 white. 



This experiment must now be interpreted. The following hypothesis 

 has been advanced with regard to it : The progeny of the first genera- 

 tion, those which were all gray, were formed by the fusion of a 

 gamete including a potency of the gray character with another 

 gamete including a potency of the white character; now these two 

 potencies pass into all the cells of the body, including the genital 

 SM 1900 22 



«)«)0 



GG GB GB BB 



Fig. 1.— Diagram of a crossing between a 

 gray mouse (black circle) and a white 

 mouse (white circle). The covering of 

 two-thirds of the white circle by the black 

 one indicates that in the hybrids the gray 

 character is dominant. 



