VARIATION AND HEREDITY 19 



whether two unlike cells meet and develop into a blue 

 chick, or two like cells join to form either a pure black 

 or a pure white bird. We may perhaps regard the 

 mixture of strains, which we find in the germ cells of 

 the hybrid blue fowl, as analogous to the mixture of 

 black and white threads which make up certain grey 

 cloths. To the eye the cloth looks grey, but in reality 

 there has been no true mingling. The threads may be 

 picked out again into a heap of white ones and a heap 

 of black. In the Andalusian fowl, black and white are 

 definite Mendelian properties, which are segregated 

 from each other in the germ cells and never mix. Not 

 all properties show these phenomena. In some cases 

 the hybridization seems to extend to the germ cells 

 themselves, and Mendel's principles do not apply. But 

 the possibility of such simple relations should never be 

 lost sight of, when examining data of inheritance. More- 

 over, some cases of mixed heredity may be explained by 

 the simultaneous action of several pairs of qualities, each 

 alone definitely Mendelian, which, acting together, are 

 difficult to disentangle. 



The Andalusian fowl illustrates the simplest form of 

 Mendelian principles. In other cases the hybrid, which 

 corresponds with the blue bird in the fowls, resembles 

 outwardly one or other of the parents, and only betrays 

 its hybrid nature in the appearance of its descendants. 



For instance, among the characters of green peas 

 studied by Mendel himself was the height of the plant. 

 Crossing a tall with a dwarf pea, he found that all the 

 resulting seeds gave rise to tall hybrid plants. In these 

 hybrids the tall character is said to be dominant, and 

 the dwarf character recessive. But the hybrid tall 



