FACTS OF INHERITANCE 135 



practical men put money on the influence of a 

 previous sire on offspring subsequently borne 

 by the same mother to a different father. More 

 serious, however, is the wide-spread scepticism as 

 to the transmission of individually acquired charac- 

 ters or modifications. 



(6) Mendelism. But the greatest change that 

 has come about since Darwin's day is the most 

 recent l one associated with the work of Mendel. 

 We shall devote some attention to this at a later 

 stage in our exposition, but it may be noted, 

 in the meantime, that Mendelian experiment 

 has afforded evidence that an inheritance often 

 consists, in part at least, of well-defined, non- 

 blending " unit characters/' " By a unit character 

 in the sense of Mendel's law we mean any quality 

 or part of an organism, or assemblage of qualities 

 or parts, which can be shown to be transmitted 

 in heredity as a whole and independently of other 

 qualities or parts." 2 The inheritance in a fertilised 

 egg-cell consists of an assemblage of distinct 

 ingredients in duplicate, contributed from the 

 father and from the mother. If both the germ- 

 cells (egg-cell and sperm-cell) bring in a similar 

 ingredient when they unite in fertilisation, then 

 all the germ-cells of the offspring will have it ; 

 if neither bring it in, then none of the germ-cells 

 of the offspring will have it. Two blue-eyed 

 parents (without pigment in the front of the 

 iris) do not have dark-eyed children. If the 



1 We must say recent, for although Mendel died two years after 

 Darwin and published his great discovery in 1865, his work was lost 

 sight of till 1900, when Correns, Tschermak, and De Vries were 

 independently led to a rediscovery of Mendel's law and to a dis- 

 covery of his buried memoirs. 



2 W. E. Castle, in " Fifty Years of Darwinism " (1909), p. 146, 



