APPENDICES 1447 



as intact entities or not at all. When two parents, belonging to the 

 same species, differ in, let us say, a pair of contrasted or "alter- 

 native" characters, the offspring will take after one parent only, as 

 regards that particular character. What is expressed in the develop- 

 ment of the hybrids Mendel called a "dominant" character; what 

 remains latent he called "recessive"; but it is not yet known why 

 one character should be dominant and its alternative recessive. If 

 the hybrids pair among themselves, or with others of similar 

 history, then the second filial generation will typically show a 



'- (3 (3<^(3 



AA AA " Aa Aa aa AA Aa Aa aa aa 



Fig. 202. 



Mendelian Inheritance in Wood Snails [Helix nemoralis). After Lang. The 

 parents (P) are pure-breeding bandless (A) and banded [a) stocks of the Wood 

 Snail. The progeny (FI) are all bandless (Aa), the bandless character 

 being therefore called dominant. In the next generation (FT I), from the 

 inbreeding of FI, there are bandless and banded forms in the ratio 3:1. 

 The bandless forms are pure banded (AA) and impure dominants (Ar?) 

 in the ratio 1:2. The pure bandless (AA), bred with others of similar 

 history, will yield only bandless. Similarly the pure banded or extracted 

 recessives [aa), if bred with others of similar history, will yield only 

 banded forms {aa). The impure dominants (Aa), bred together, will show 

 in their progeny the lAA -f 2Aa -\- laa ratio. See p. 11 00. 



quarter pure-breeding "dominants", a quarter pure-breeding 

 "recessives", and a half "impure dominants" like the parents. 

 Moreover, Mendel suggested how this comes about by the segregation 

 of the germ-cells of the hybrids into two contingents, one-half carry- 

 ing the factor or "gene" for the dominant character, and the other 

 half carrying the corresponding initiative for the recessive character. 

 The impulse to further research on these lines since 1900 has been 

 great and fertile. 



An influential event was the discovery of the dual nature of lichens 

 by De Bary (1866) and Schwendener (1868), who showed that these 

 familiar encrusting plants consist of an Alga and a Fungus living 



