THEORY OF EVOLUTION 41 



1. Possess constant differentiating characters. 



2. The hybrids of such plants must, during the 

 flowering period, be protected from the influence of all 

 foreign pollen, or be easily capable of such pi'otec- 

 tion." 



Why do biologists throughout the world to- 

 day agree that Mendel's discovery is one of 

 first rank? 



A great deal might be said in this connec- 

 tion. What is essential may be said in a few 

 words. Biology had been, and is still, largely 

 a descriptive and speculative science. 3Iendel 

 showed by experimental jji-oof that heredity 

 coidd he explained by a simp)le mechanism. 

 His discovery has been exceedingly fruitful. 



Science begins witli naive, often mystic con- 

 ceptions of its problems. It reaches its goal 

 whenever it can rep)lace its early guessing by 

 verifiable hypotheses and predictable results. 

 This is what JNIendel's law did for heredity. 



Mendel's First Discovery — SEGREGATioisr 



Let us turn to the demonstration of his first 

 law — the law of segregation. The first case I 

 choose is not the one given by Mendel but one 

 worked out later by Correns. If the common 



