148 THE BEHAVIOR OF UNIT 



knowledge of heredity, and merits further con- 

 sideration. 



Mendel conceived of unit characters as exist- 

 ing always in pairs, one of which might be sub- 

 stituted for the other by suitable crosses. We 

 are now coming to realize that this is an inade- 

 quate statement of the matter. What is paired 

 is not the unit character alone, but the entire or- 

 ganism. All its characters and parts have their 

 basis in paired structures in the protoplasm of 

 the individual, one member of each pair being 

 derived from the mother of the individual, one 

 from the father. The cytologist has visible evi- 

 dence of this fact in the doubling of the number 

 of chromosomes at fertilization, and their subse- 

 quent reduction when the reproductive cells 

 ripen; the experimental breeder has evidence of 

 this duaUty equally convincing as regards many 

 hereditary characters, but the evidence is clearest 

 in the case of characters which occasionally are 

 lost. It is only in such cases that we can with 

 certainty identify unit characters. By compar- 

 ing an individual which has a certain character 

 with another individual which does not have it, 

 we learn how much that character includes, and 

 we can learn this in no other way. Experimen- 

 tal breeding will show whether the character is 

 simple, is really a unit, or is an aggregate of in- 

 dependent units. Thus if we cross a black 

 guinea-pig with one which lacks black — say a 

 brown one — ^we obtain only black offspring, but 



