HYBRIDS 



383 



are self -fertilised, the latent character reappears in a certain 

 number of the individuals arising from the resulting seeds, a 

 fact which proves that it must have been present, although not 

 outwardly manifest. 



An exact study of the phenomena of hybridisation was first 

 undertaken by Mendel, an Austrian monk, by considering the 

 behaviour of single pairs of characters only. In one of his 

 earliest experiments he crossed tall and dwarf Peas, the whole 

 of the resulting 

 offspring (first 

 generation) be- 

 ing tall. The 

 seeds produced 

 from these, by 

 self-fertilisation, 

 afforded two 

 classes of in- 

 dividuals, three- 

 quarters being 

 tall and one- 

 quarter dwarf. 

 But of these 

 tall Peas of the 

 second genera- 

 tion (if again 

 self-fertilised) 

 only one-third 

 bred true, the 

 remainder be- 

 having just like 



the original hybrids of the first generation, and giving rise 

 to tails and dwarfs in the proportion of three to one. The 

 dwarfs in every case bred true. In the second generation of 

 such a cross, then, as regards any one particular pair of charac- 

 ters, half the offspring are pure (one quarter resembling one 

 parent, another quarter the other parent), whilst the other half 

 are impure and bear the characters of both. Representing the 

 tall Pea by T and the dwarf Pea by d, these results can be 

 graphically expressed as follows : 



FIG. 223. Twigs of A, ordinary Beech, and B, cut- 

 leaved Beech, illustrating a vegetative mutation. 



