372 THE STUDY OF ANIMAL LIFE CHAP. 



There are some curious cases, probably admitting of 

 Mendelian explanation, where the offspring show what 

 may be called a coarse-grained combination of the paternal 

 and the maternal characteristics, the former appearing 

 in one part of the body, the latter in another part, as 

 when a light-coloured horse and a dark-coloured mare 

 have a piebald foal, or when a sheep-dog has an eye like 

 its father's on one side of the head, and an eye like its 

 mother's on the other side. This is often described as 

 particulate inheritance. 



The mode of inheritance which is now called Mendelian 

 was discovered in 1865 by Gregor Mendel (1822-1884), 

 an Austro-Silesian abbot, who worked chiefly with the 

 edible pea, Pisum sativum, which has many well-marked 

 varieties, and is habitually self-fertilised. Mendel's work 

 remained practically unknown till 1900, when De Vries, 

 Correns, and Tschermak independently, and almost simul- 

 taneously, reached experimental results closely resem- 

 bling Mendel's. The inquiry has been continued by many 

 investigators, notably Bateson, Punnett, Castle, Cuenot, 

 Morgan, and Davenport. 



When Mendel crossed a pure-bred giant variety of pea 

 with a pure-bred dwarf variety, the offspring were all 

 tall. The character of tallness which appeared in the 

 hybrid generation (Fj), to the exclusion of dwarfness, was 

 called by Mendel the ' dominant ' character, the other 

 being " recessive." 



The tall cross-bred or hybrid peas were left to self- 

 fertilise, which corresponds to close inbreeding or pairing 

 of similars in animals, and in their progeny there were 

 tails and dwarfs in the average proportions of 3 : 1 ; 75 per 

 cent, tails and 25 per cent, dwarfs in the second filial 

 or F 2 generation. 



When the dwarfs of this F 2 generation were left to 

 self-fertilise, their offspring in the F 3 generation were all 

 dwarfs, and further generations bred from them also con- 

 sisted exclusively of dwarfs. They may be called pure 

 or extracted recessives, being " pure " as regards dwarf- 

 ness. 



But when the tails of the F 2 generation were left to 



