18 HEREDITY AND INHERITANCE 



may be has been a mulatto who inherits some of his father's 

 intellectual virtue and some of his mother's physical strength, 

 including, for instance, a peculiar insusceptibility to yellow 

 fever. Here are complex problems of inheritance. How is 

 it that certain characteristics of the son are almost wholly 

 of paternal origin, while in other respects he takes after his 

 mother ? 



An English sheep-dog may show a paternal eye on one side of 

 the head, a maternal eye on the other. A piebald foal may 

 have its mother's hair on some patches, its father's hair on 

 others. Such cases raise the problem of the different modes of 

 hereditary resemblance, of the mosaic-like constitution of an 

 inheritance, and of the various ways in which this may find 

 expression in development. 



Given in our British population a thousand fathers six feet 

 high, we can predict with great accuracy the average height of 

 their sons. Though we cannot make any prediction as to an 

 individual family, we know that the average height of all the 

 sons of these tall men will be nearer the average height of the 

 total male population than the height of six feet is. We know, 

 however, that the tall do not always beget the tall, nor the 

 small the small ; that stature in mankind is a character that 

 blends ; and that even among the sons of the thousand fathers 

 we have spoken of, there will be every gradation between the 

 tallest and the smallest. How different this is from stature in 

 pure-bred peas, for if a tall variety of pea be crossed with a 

 dwarf, all the offspring are tall, and among their offspring 

 in turn three-fourths are tall and one-fourth dwarf, but none 

 between the two. 



White fowls crossed with black ones often have white off- 

 spring ; black guinea-pigs crossed with white ones have black 

 offspring ; black-eyed white guinea-pigs crossed with albinos 

 have black offspring. It seems at first sight arbitrary, but a 

 rational interpretation of each of these results has been given. 



