232 SECRETS OF ANIMAL LIFE 



mak; and more progress has been made with the 

 science of heredity since the century began than in 

 all previous years. For this we have to thank numer- 

 ous investigators among whom Bateson stands pre- 

 eminent. What are the central ideals of Mendel- 

 ism? How far does it apply? What practical 

 promise does it offer? 



There are three fundamental ideas in the Men- 

 delian conception of inheritance : i. The first is the 

 idea of " unit characters." An inheritance is, in part 

 at least, built up of numerous more or less clear-cut, 

 crisply defined, non-blending characters, which are 

 continued in some of the descendants as discrete 

 wholes, neither merging nor dividing. If a man 

 has his ringers all thumbs, i.e., with two joints 

 instead of three, this unit character of " brachy- 

 dactylism " is sure to be continued in a certain 

 proportion of his descendants. Night-blindness, or 

 the inability to see in faint light, has been traced 

 through a lineage since near the beginning of the 

 seventeenth century. A definite type of very 

 intelligent dwarf has been known to reappear for 

 four or five generations. The persistence of the 

 Hapsburg lip is a familiar instance of the way in 

 which a unit character comes to stay. These unit 

 characters behave as if they were discrete entities 

 which can be shuffled about and distributed to the 

 offspring to some degree independently of one an- 

 other. Some suppose that they are represented by 

 specific particles in the germinal material; others 

 would not go further than saying that they are 



