GENERAL TOPICS IN INHERITANCE. 8 1 



quoted above : ' ' The characteristics of organisms are built up of units that 

 are sharply separable one from another," and " Transitions exist between 

 the unit as little as between the molecules. ' ' Single comb is one unit and 

 pea comb is a different unit, but they are not sharply separable. Crest and 

 no crest are units, but they run into each other in hybridizing. Unit char- 

 acters may show transitions, and, if so, they may have originated gradually, 

 so far as I see. It does not follow that they must have originated gradually. 



ALTERNATIVE, PARTICULATE (MOSAIC), AND BLENDING INHERITANCE. 



Doubtless Darwin's statement that crossed characters usually blend is 

 still the prevalent view. Much if not most biometric work in heredity has 

 been made on this basal assumption. I may say that I began my experi- 

 ments prejudiced in favor of this view. 



The results that have been recorded in the foregoing pages indicate that 

 probably in general typical blending of characters is rare. Excepting char- 

 acters like general form of the body, which are doubtless not units, but 

 complex, I have, indeed, seen no single case of a typical blend. A fusion 

 of characters is a rather rare phenomenon. Human skin color is the one 

 striking case. One can but wish we had more careful data on inheritance 

 of human skin color in successive generations. Other human characteristics 

 show alternative inheritance. This is strikingly true in Farabee's family of 

 hypodactyls cited above. It is said to be true of eye color and probably 

 of the states of general pigmentation known as blonde and brunette. 

 The following characters of poultry show alternative inheritance : 



Comb form. Uropygium. Earlobe color. 



Nostril form. Tail length. General plumage color 



Cerebral hernia. Vulture hock. (sometimes). 



Crest. Booting. Color of hackles. 



Muff. Extra toe. Wing bar. 



Beard. Color of mandible Shafting. 



Frizzling. and foot. Body lacing. 



Silkiness. Iris color. Penciling. 



The following characteristics show particulate inheritance : 



Iris color (sometimes?). 



White and black, producing barring (Series VI, VII, IX, XII). 



White and black, producing blue (fig. 54) (a fine mosaic of white and 



black). 



It is too early yet to interpret the cases of particulate inheritance. It is a 

 striking fact that, excepting the Tosa X White Cochin cross, all my barred 

 birds reared to maturity had the White Leghorn Bantams as mother or 

 father. Now, as repeatedly observed, these bantams were heterogametous. 

 It is possible that they contain barred blood in the ' ' fixed ' ' condition. 

 Aside from the fact that they throw a certain proportion of barred birds, this 

 conclusion gains support from the fact that the wing coverts of the male are 

 6 



