STATISTICAL AND PHYSIOLOGICAL LAWS 331 



offspring are all tall. Tallness is the " dominant " character and 

 dwarf ness is " recessive." The tall hybrids are allowed to self- 

 fertilise, which comes to the same thing as inbreeding, and the 

 next generation show 75 per cent, tails and 25 per cent, dwarfs. 

 The dwarfs if self-fertilised will produce only dwarfs ; they are 

 pure from all taint of tallness. Of the tails, one-third will 

 produce only tails when self-fertilised ; they are pure from all 

 taint of dwarfness. The remaining two-thirds will produce 

 when self-fertilised the same proportion of 75 per cent, tails and 

 25 per cent, dwarfs. They may be called impure tails. Thus 

 the result is: 



Tall x Dwarfs = Parents 



I 

 All Tails = Hybrid offspring 

 J- 



25% Pure Tails + 50% Impure -f 25% Pure = Next generation 



Tails Dwarfs 



As we shall afterwards see, the result thus briefly summarised 

 — a statement of fact — was interpreted by Mendel in terms of a 

 simple theory, viz. that the germ-cells of the hybrid offspring 

 are segregated into two sets of pure gametes, each set bearing 

 in potentia one of the contrasted characters of the original tall 

 and dwarf parents. If so, the chances of fertilisation must give 

 the result indicated. Now the point is, that when we are dealing 

 with Mendelian unit characters, such as tallness and dwarfness 

 in peas, Galton's law is not relevant, whereas when we are dealing 

 with non-Mendelian characters, such as the ordinary fluctuations 

 of stature in mankind, Galton's law is a valuable statistical 

 description. 



What are called Mendelian phenomena are illustrated when 

 the parents differ in sharply defined contrasted characters 

 which cannot blend or compromise, and the extension of ex- 

 periment will doubtless go on increasing our knowledge of these 

 unit characters and their behaviour. The formulation will 



