1 8 HEREDITY AND INHERITANCE 



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. 



De Vries (1903) tells us of a well-known shrub of the hybrid 

 Adam's laburnum {Cytisus adami), which grew in the village 

 of Bloemendael, in Holland. This hybrid is a cross between the 

 common laburnum {Cytisus laburnum) and another species 

 of the same genus, Cytisus pitrpureus, and has some traits of 

 both. It is, however, absolutely sterile, and is multiplied by 

 grafts. The tree in the village was particularly interesting, 

 for it bore three kinds of flowers, — some pink, others large and 

 yellow, others small and purple. That is to say, it bore its own 

 hybrid flowers, and also those of its two parents, and the leaves 

 and ramifications of the parts of the tree which bore these three 

 kinds of flowers were likewise of the same three kinds, and could 

 be distinguished even in winter. In other wcrds, in the same 

 organism there were three kinds of characters, which could be 

 separated out from one another in the course of growth. The 

 characters of the two parents may combine in a close companion- 

 ship, but when certain conditions arise the companion-characters 

 may separate and each set may pursue its own path. It is an 

 intricate problem to study the relation of a hybrid's characters 

 to those of its parents. 



In Chapter X. we shall have to allude to many problems like 

 the following : 



