THE HISTORICAL DEVELOPMENT OF BIOLOGY 543 



them himself, so knew exactly the ancestry of each new set of 

 descendants. After years of patient experiment and careful record 

 he reached some conclusions. He found that if he crossed tall 

 with short that the next generation were hybrids but tall in ap- 

 pearance, that is, tallness had overcome shortness as a char- 

 acteristic in that generation. 



Many characteristics were found to be stronger at first and 

 were called " dominant " characteristics. Those which were 

 crowded out were called " recessive." However when these 



O 



P 



FIG. 168. Diagram to show the segregation and re-combination of the 

 factors (black and white) in the gametes, and the presence of both in the hybrid 

 F'. (From Morgan, see Calkins.) 



hybrids were bred together both the original characteristics re- 

 appeared in a constant proportion of tall, short, and tall hybrids. 



The reason is that the two characteristics remained separate 

 in the hybrids and did not blend, hence when hybrid was bred with 

 hybrid the next generation would combine these characteristics 

 according to the mathematical law of probabilities or chance. 



To illustrate, let x and y stand for any two non-blending char- 

 acteristics. The first crossing would produce hybrid offspring 

 having xy characteristics, but if x were dominant, y would not 

 appear. 



