How Darwinism Stands To-Day 881 



If a man has his fingers all thumbs, i.e. with two joints 

 instead of three, this peculiarity (called "brachydactylism") is 

 sure to be continued in a certain proportion of his descendants; 

 and we call it a "unit-character." The persistence of the Haps- 

 burg lip in the Royal Houses of Austria and Spain is a good 

 instance of how a unit-character comes to stay for many genera- 

 tions. Night-blindness, or the inability to see in dim light, has 

 been traced through a lineage since near the beginning of the 

 seventeenth century another illustration of the persistence of a 

 unit-character. We do not precisely know what the germinal 

 factors of the unit-characters are like, but in some cases it is 

 known that they lie in linear order in the nuclear rods or chromo- 

 somes. In some instances (though it is impossible in a few 

 words to explain how) we know what region of the chromosome 

 the factor occupies. But the most important point is that the 

 unit-characters (or their factors) behave as if they were definite 

 entities, like the radicals in chemistry, which can be shuffled 

 about and distributed to the offspring in some degree inde- 

 pendent of one another. Thus in the lineage of the "night- 

 blind" it was not every individual that showed the peculiarity, 

 but only a certain proportion in each generation. 



In his masterly book on Mendelism Professor R. C. Punnett 

 refers to a unit-character as follows: "Unit-characters are rep- 

 resented by definite factors in the gamete [or germ-cell], which, 

 in the process of heredity, behave as indivisible entities, and are 

 distributed according to a definite scheme. The factor for this 

 or that unit-character is either present in the gamete or it is not 

 present. It must be there in its entirety or be completely 

 absent." 



6 



The second fundamental idea in Mendelism is that of domi- 

 nance. When Mendel crossed a pure-bred tall pea with a pure- 

 bred dwarf pea the offspring were all tall. So he called the 



