HEREDITY IN PLANTS, ANIMALS, AND MAN. 375 
trated by eye-colour in the American fruit fly Drosophila. The wild fly 
has red eyes, and a sport or mutant is known which has white eyes. Ifa 
white-eyed male is mated with a red-eyed female all the offspring are red- 
eyed, and males and females occur in equal numbers. When these hybrids 
are mated together there result three red-eyed flies and one with white 
eyes—the usual Mendelian proportions. The white-eyed flies, however, 
in this generation are all males, like the grandfather. If however we 
make the original cross in the opposite way, mating a red-eyed male with 
a white-eyed female a different result is obtained. Instead of having all 
red-eyed children, males and females in equal numbers, we have equal 
numbers of white-eyed males and red-eyed females. In the next genera- 
tion also the result is different, for when one of these red-eyed females is 
mated with a white-eyed male the offspring are red-eyed females, red- 
eyed males, white-eyed females and white-eyed males in equal numbers. 
It would carry us too far were I to attempt to give the explanation which 
has been put forward to account for this, so [ shall content myself with 
stating the facts to show that the simple Mendelian law may at times 
seem to give highly complex results. 
That knowledge gained by Mendelian investigations may be of great 
value to practical agriculture is shown by Prof. Biffen’s work on the 
varieties of wheat. The wheats usually grown in England produce heavy 
crops, but the flour obtained from them 1s not satisfactory from a baker’s 
point of view. A loaf made from this flour does not rise well when baked. 
In order to correct this it is usual to mix the English flour with flour from 
a so-called “ hard ” foreign wheat, which contains a larger proportion of 
gluten. English wheats, also, are very liable to a disease known as “ rust,” 
which is caused by the growth of a fungus on the plants. Prof. Biffen 
was able to show that good cropping power, hardness, and ability to 
resist rust are all characters which behave in a Mendelian way. By a 
long series of experiments in crossing different varieties of wheat he was 
able to produce a variety which possessed good cropping powers, the hard 
qualities of foreign wheat, and also a complete power of resisting rust. 
This wheat can be grown quite successfully in the English climate, and it 
has kept its special qualities unchanged for a number of years. 
And now for a few minutes we will direct our attention to the question 
in connection with this subject of Mendelian heredity, which is perhaps 
of more interest to us than any other. In the human race does inherit- 
ance take place in accordance with Mendel’s law? There is considerable 
evidence that certain characters do follow this law and that the same 
thing is true of certain diseased conditions. 
The inheritance of eye-colour is a striking instance, which was investi- 
