THEORETICAL INTERPRETATION 345 



purple or white flowers (in each case, the dominant character has 

 been named hrst). Let us assume that these are pure-bred 

 varieties, well-established and breeding true, the tall form always 

 producing tall offspring, the dwarf form always producing dwarf 

 offspring, and so on. Let us also assume that the germ-cells 

 contain material representatives of these " unit characters " — 

 tallness, dwarfness, rounded seeds, angular seeds, yellow cotyle- 

 dons or green cotyledons, purple flowers or white flowers. 



The egg-cell of the tall pea is normally fertilised by a pollen- 

 grain from the same pea, and the fertilised egg-cell develops into 

 an embryo which becomes a tall pea. As the varieties breed 

 true we assume that the only quality affecting dimensions which 

 the germ-cells bear (in expressible strength, at least) is the quality 

 of tallness. 



But let us now take the case of a tall pea pollinated from 

 a dwarf pea. The offspring become tall peas — the parent with 

 the dominant character is prepotent. But the fertilised egg- 

 cells which gave rise to these tall peas must have contained not 

 only representative primary constituents corresponding to the 

 quality of tallness ; but also representative primary constituents 

 corresponding to the quality of dwarfness. This quality of 

 dwarfness is not expressed in development, but it must be 

 present, as subsequent generations show ; for when the egg- cells 

 of the hybrids are self- fertilised they develop into offspring 

 partly tall and partly dwarf. What Mendel suggested was that 

 the hybrid produces in equal numbers two kinds of germ-cells 

 (two kinds of egg- cells or two kinds of pollen-grains) — that there 

 is in the developing reproductive organ a segregation of germ- 

 cells into two equal camps, one camp with the potential " fac- 

 tor " of tallness, the other camp with the potential " factor " 

 of dwarfness. If there be six ovules, three have in their egg- 

 cell the primary constituent or factor corresponding to tallness, 

 and three contain the primary constituent or factor of dwarf- 

 ness. Each of these is pollinated by a pollen-grain, which, by 



