1360 PHYSIOLOGY 



characters were present together in one plant it would partake of the 

 dominant type ; the fact that this plant possessed the recessive 

 character would only be shown by the results of breeding. In the 

 case of the peas the tall character was dominant over the dwarf. Thus 

 when the tall and dwarf pea were crossed the first generation of plants 

 would exhibit the dominant character and be tall. In the second 

 generation, however, 25 per cent, of the individuals would be pure 

 dominants (D + D), 25 per cent, would be pure recessive (R -j- R), 

 while 50 per cent, would be mixed (D -+- R). The pure dominants 

 bred together would always give rise to nothing but pure dominants, 

 the recessive to recessive, while the mixed type would always, as before, 

 give rise to 25 per cent, pure dominants, 50 per cent, mixed, and 

 25 per cent, pure recessives. These results may perhaps be made 

 clearer by the following Table : 



D + R 



25% D 



It has been suggested that a very large number, if not all, of the 

 characters of an individual might be brought under this law. This 

 might be done by indefinitely subdividing the characters, but the 

 question would then become beyond the limits of analysis or experi- 

 mental investigation. There is no doubt that many qualities are 

 subject to Mendel's law, and that their study will be of considerable 

 assistance in guiding the efforts of our breeders and horticulturists in 

 the formation of new varieties desirable for their value to man. In 

 respect of many qualities the Mendelian law seems to fail. Thus in 

 man the progeny of a cross between a white and black race are more or 

 less intermediate between the two and vary according to the amount of 

 black and white blood introduced in succeeding generations. Definite 

 black and white individuals are not produced, but merely individuals 

 of various degrees of brownness. 



