234 SECRETS OF ANIMAL LIFE 
illustration of characters that exhibit Mendelian 
inheritance, the following may be cited, the domi- 
nant condition being named first in each case: Horn- 
lessness and the presence of horns in cattle, normal 
hair and long “ Angora” hair in rabbits and guinea- 
pigs, kinky hair and straight hair in man, crest and 
no crest in poultry, extra toes in poultry and the 
normal number four, bandless shell in wood-snail 
and banded shell; yellow cotyledons in peas and 
green ones, round seeds in peas and the wrinkled 
form, absence of awn in wheat and its presence, 
susceptibility to rust in wheat and immunity to 
this disease, two-rowed ears of barley and six-rowed 
ears, markedly dentate margin in nettle leaves 
and slightly toothed margin. Why one character 
should be dominant and another recessive is not 
known; a positive feature may be recessive, and 
a negative feature may be dominant. It should be 
noted that in a large number of cases of alternative 
or Mendelian inheritance the dominance in the 
offspring is not complete; thus, if black Andalusian 
fowls be crossed with white ones the progeny are 
“blue” Andalusians. 3. The third idea in Mendel- 
ism is the theory of segregation. Mendel supposed 
that the hybrid or cross-bred offspring produced two 
kinds of germ-cells in approximately equal numbers 
—one-half with the determiner or factor corre- 
sponding to the dominant character, and the other 
half without it, or with the factor corresponding to 
the recessive character. In other words, each germ- 
cell is “pure” with respect to any given unit 
