106 Heredity and Environment 



Sometimes dominance is incomplete in early stages of develop- 

 ment but becomes complete in adult stages. Davenport found that 

 when pure white and pure black Leghorn fowls are crossed the 

 chicks are speckled white and black, but in the adult fowl domi- 

 nance is complete and the plumage is black. Similar conditions of 

 delayed dominance are well known in the color of hair and eyes 

 of children, though dominance may become complete when they 

 have reached adult life. 



Reversible Dominance. — In a few instances a character may be 

 dominant at one time and recessive at another. Thus Davenport 

 found that an extra toe in fowls is dominant under certain cir- 

 cumstances and recessive under others. Tennent found that char- 

 acters which are usually dominant in hybrid echinoderms may be 



• made recessive if the chemical or physical nature of the sea water 

 is changed. Such cases seem to show that dominance may depend 

 sometimes upon environmental conditions,' sometimes upon a 

 particular combination of hereditary units. 



Dominance Not Fundamental. — In all cases dominance means 

 merely the development in offspring of certain characters of one 

 parent, while contrasting characters of the other parent remain 

 undeveloped. The appearance of any developed character in an 

 organism depends upon many complicated reactions of germinal 

 units to one another and to the environment. Under certain con- 

 ditions of the germ or of the environment some characters may 

 develop in hybrids to the exclusion of their opposites whereas 

 under other conditions these results may be reversed or the char- 



♦ acters may be intermediate. The principle of dominance is not a 

 fundamental part of Mendelian inheritance. Even when the 

 characters of hybrids are intermediate between those of their 

 parents, if the parental types reappear in the F„ generation we 

 may be certain that we are dealing with cases of Mendelian 

 inheritance. 



3. The Principle of Segregation. — The individuality of inheri- 

 tance units, and their segregation or separation in the sex cells 

 and recombination in the zygote are fundamental principles of 



