APPENDIX B 357 



acquirement by virtue of a power with which evolution has 

 endowed his race; but the child makes it in quite another 

 fashion by virtue of a mystic power with which evolution has 

 not endowed the race, but which he develops suddenly. 1 



APPENDIX B 



MENDEL'S LAWS, AND THE MUTATION THEORY OF EVOLUTION 



(Seep. 75.) 



IN the year 1865 Gregor Mendel, Abbot of Briinn, published 

 the results of some interesting and important experiments. He 

 crossed varieties of the pea. The flower of this plant is not 

 adapted to receive the visits of insects, and therefore is normally 

 self-fertilized, the pollen from the anthers falling on the stigma 

 of the same flower. Mendel amputated the stamens and fertilized 

 with pollen conveyed from a distinct variety. Thus he crossed 

 dwarf peas with the ordinary tall type. The first hybrid 

 generation consisted solely of tall plants, which were allowed to 

 self-fertilize themselves, as were their descendants. The next 

 generation broke up into tall and dwarf plants, in the average pro- 

 portion of seventy-five per cent, of the former and twenty-five of 

 the latter. No plants were intermediate in height. Subsequently, 

 these dwarf plants produced only their like, and continued to do 

 so as long as the experiment was carried on. It seemed evident 

 that their race was purged of the influence of the tall variety. 

 A similar number of tall plants behaved in a like manner. 

 Their race also had become " pure." But the remaining plants 

 (50 per cent.) behaved like their cross-bred parents, producing 

 tall and dwarf offspring in the old proportion, the dwarfs being 

 all pure, and the tall plants pure and impure. This process, also, 

 continued indefinitely. 



Mendel named the "tall" character dominant and the "dwarf" 

 recessive, and supposed that when the "units" which represented 

 them in the germ-cells met in the fertilized ovule, the dwarf unit 

 became latent, leaving the other to direct the development of 

 the plant. He supposed, also, that, during the formation of 

 the germ-cells of this plant, a separation of units took place, so 

 that each pollen-grain and each unfertilized ovule received a 

 single dominant unit, or a single recessived unit, but never a 

 blended unit nor both kinds of units. Whence it followed that 

 when a pollen-grain containing a dominant unit fertilized an ovule 

 containing another dominant unit, the plant that resulted was a 

 pure dominant. Pure recessives arose in a similar manner. But 

 1 See pp. 248-50. 



