CASTLE. — MENDELS LAW OF HEREDITY. 543 



on one of two assumptions. Either the germ-cells bear the two charac- 

 ters in the balanced relationship, ^ AB, as do spotted mice ordinarily, or, 

 of the two gametes which unite in fertilization, one invariably bears the 

 character A, the other the character B. 



3. Coupled characters. This is the phenomenon of correlation of 

 characters in heredity. It is sometimes found that, in cross-breeding, two 

 characters cannot be separated. When one is inherited, the other is 

 inherited also. Thus, in crossing different sorts of Datura (the James- 

 town weed) it has been found that purple color of stem invariably goes 

 with blue color of flowers, whereas green stems are constantly associated 

 with white flowers. Again in mice, rabbits, and most other mammals, 

 Avhite hair and pink eyes occur together and may not be separated in 

 heredity. Very rarely, however, as I have observed, an otherwise per- 

 fectly white guinea-pig has dark eyes ; further the ordinary albino guinea- 

 pig with pink eyes has usually smutty (brown-pigmented) ears, nose, and 

 feet. These exceptional conditions probably represent stable couplings 

 of a pccrt only of the dominant character (pigmented coat) with the reces- 

 sive character (white coat), and are similar in kind to the i DR character 

 of the spotted mice. For guinea-pigs do occur entirely devoid of the D 

 character, i. e. without dark nose, ears, and feet, and with pink eyes. 

 These doubtless represent the pure recessive condition. 



Further, coupling may occur between a number of characters greater 

 than two, so that they form, to all intents and purposes, in heredity, one 

 indissoluble compound character. Thus, Correns (:00) observed that 

 in crosses between two species of stocks (Mathiola incana DC. and M. 

 glabra DC.) the second generation hyl)rid3 showed reversion to one or 

 the other of the parental forms in all three of the principal differential 

 characters studied, viz., hairy or glabrous stems, violet or yellow-white 

 flowers, and blue or yellow seed. A blue seed always produced a hoary 

 plant bearing violet flowers ; a yellow seed always produced a glabrous 

 j)lant bearing yellow or white flowers. 



4. Disintegration of characters. This is the converse of the foregoing 

 process. Not only may characters apparently simple be coupled together 

 in heredity to form composite units of a higher order, but characters 

 whi(;h ordinarily behave as units may as a result of crossing undergo dis- 

 integration into elements separately transmissible. Thus the gray coat- 

 color of the house-mouse is always transmitted as a dominant unit in 

 primary crosses with its white variety ; but in the second cross-bred 

 generation a certain number of hhtck mice appear, some or all of which 

 are probably hybrids. For similar black mice obtained by crossing black- 



