224 THE PERPETUATION OF ADAPTATIONS 



on self-fertilization, produces pure whites, pure reds, and the 

 composite pinks. 



The same result may be worked out theoretically along the 

 lines of Weismann's hypothesis of the significance of maturation. 

 The chromosomes which unite at pseudo-reduction contain 

 allelomorphs which are separated from one another during the 

 maturation divisions (Fig. 90). Two kinds of spermatozoa 

 and two kinds of eggs result. On self-fertilization, one kind of 

 sperm may unite with an egg containing its like kind, or it may 

 unite with an egg containing its allelomorph. Or the other 

 sperm may unite with its allelomorph or its like, the result being 

 the Mendelian proportion of three to one. 



B. HEREDITY OF Two PAIRS OF CHARACTERS. Mendel 

 worked out the principles of heredity in cases where two or 

 more pairs of characters are involved, and found the same under- 

 lying principles of dominance and segregation as in the case of a 

 single pair. He crossed a pea producing yellow and round 

 seeds, with one producing green and wrinkled seeds. The Fi 

 generation or hybrid seeds were yellow and round, showing 

 that the round characteristic is dominant over the wrinkled. 

 The Fi plants, when self-fertilized, produced some yellow and 

 round peas, some yellow and wrinkled, some green and round 

 peas, and some green and wrinkled in the proportion of 9 : 3 13:1. 

 The explanation is the same as for the simpler cases of one pair 

 of characters. One parent produced germ cells containing the 

 factors for yellow (Y) and round (R) ; the other parent produced 

 germ cells containing the factors green (G) and wrinkled (W). 

 The fertilized eggs, or Fi generation, must therefore have con- 

 tained YRGW, the allelomorphs being YG and RW. These 

 allelomorphs are separated during maturation, the germ cells 

 containing either YR, YW, GR or GW, since these are the only 

 possible combinations. If the hybrids are self-fertilized, there 

 would be four kinds of male and four similar kinds of female 

 gametes, which would give sixteen possible combinations as 

 shown in Fig. 97. 



These experiments have been so often repeated, and on so 

 many different plants and animals with many different charac- 



