BIRDS AND EVOLUTION 391 



of the ancestral rock-dove. As will be made clear in con- 

 nection with Mendelian inheritance, there is a more probable 

 explanation of what happened. 



Professor Ewart also reported that a dark bantam hen, 

 crossed with an Indian game Dorking cock, produced amongst 

 others a cockerel almost identical with the Indian jungle 

 fowl {Gallm hankiva), which is the ancestral wild stock of 

 domesticated fowls. This recalls Darwin's case of crossing 

 a white Silky hen with a black Spanish cock, for there was 

 among the progeny a cock that looked like a rehabilitation 

 of the original wild Gallus hankiva type. 



When the offspring resembles not its immediate parents, 

 but one of its grandparents, the term reversion is quite 

 inappropriate ; such a frequent occurrence as " skipping a 

 generation " admits of a ready Mendelian interpretation, as 

 we shall presently see. Let us now turn to the fifth possi- 

 bility, {e) Mendelian Inheritance. 



Mendelian Inheritance. — There are three fundamental 

 ideas in Mendelism. The first is the idea of " unit cha- 

 racters " already referred to — non-blending, crisply defined 

 characters which are continued in at least some of the 

 descendants as definite wholes, neither merging nor dividing. 

 Now, as Professor Punnett puts it, " unit- characters are 

 represented by definite factors in the gamete [or germ-cell], 

 which, in the process of heredity, behave as indivisible 

 entities, and are distributed according to a definite scheme. 

 The factor for this or that unit-character is either present in 

 the gamete or it is not present. It must be there in its 

 entirety or be completely absent." 



The second fundamental idea in Mendelism is that of 

 dominance. When a crested fowl is crossed with one 

 without a crest, the offspring have crests ; the crest is called 

 the dominant character and the absence of a crest is called 

 recessive. But what makes one character doininant and 

 another recessive we do not know. The character of having 

 extra toes in poultry is dominant, and that of having normal 

 toes is recessive. Broodiness is dominant, its absence 

 recessive. Rose comb and Pea comb in fowls are dominant, 

 while a normal comb is recessive. 



