62 



EVOLUTION AND GENETICS 



Fig. 20. — Diagram to show the distribution of the genes 

 for white and for blaclv in the Andalusian cross. (See 

 iig. 19.) 



these hy'brids self -fertilize (or are bred to each 

 other) there are two kinds of offspring, tall and 

 short, produced in the ratio of three to one. The short 

 peas, if self-fertilized, breed true {jig. 22), but if 

 the tall peas are self-fertilized they are found to be 

 of two kinds — one-third of them breed true and two- 

 thirds of them produce three tall to one short off- 

 spring {jig. 22) . It is obvious that here, as in the four 

 o'clock and in the Andalusian fowl, there are pres- 

 ent in the second generation three kinds of offspring 

 in the proportion of one j^ure tall, to two hybrid 

 tails, to one pure short. It is the discovery by Men- 

 del of the existence of these three kinds of indi- 

 viduals in the second generation that enabled him 

 to deduce his first law. He discovered that there 



