DEVELOPMENT AND HEREDITY 1215 



extent in every chromosome ? In the case of many qualities, especially 

 those which distinguish the species as apart from the individual variation 

 or family characteristic, we must probably accept the latter idea as correct. 

 In this case the child can be regarded as representing an arithmetical mean 

 of both its parents. In certain respects, however, a quality seems to be 

 transmitted from parent to offpsring either completely or not at all. This 

 is specially applicable to those characteristics which have been rapidly 

 produced by artificial selection, characters which, if artificial selection be 

 abandoned, rapidly disappear, with reversion to the type from which the 

 special strain was ultimately produced. The way in which these charac- 

 teristics are transmitted was first studied by Mendel and has been formulated 

 as Mendel's law. Mendel's first experiments were carried out on peas. 

 On crossing a tall plant with a dwarf plant seeds were obtained from which 

 all the plants were tall. On recrossing the plants of this generation among 

 one another a third generation was obtained in which 25 per cent, of the 

 plants were dwarfs and 75 per cent, were tall. Crossing the dwarf plants 

 among themselves led to the production of dwarf plants through successive 

 generations. Of the tall plants 25 per cent, and all their descendants con- 

 tinued to produce tall plants when self -fertilised, whereas of the remaining 

 50 per cent, of the tall plants 25 per cent, produced dwarfs and the remaining 

 75 per cent, produced tall plants. On continuing the process of breeding, the 

 dwarf plants when self -fertilised always produced dwarfs, whereas of the 

 tall plants 25 per cent, produced tall plants, which bred true, while the 

 remaining 50 per cent, produced the same percentage of tall and dwarf as in 

 the preceding generations. Mendel explained these results by the assump- 

 tion that a character could be dominant or recessive. If both 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 + 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 reces- 

 sives. These results may perhaps be made clearer by the following Table : 



D + R 



I 

 DR 



25% 



D 



