concentrating his attention upon a single pair of alternative or
contrasted characters at a time, and observing how these alternative
characters are transmitted.

Thus when he crossed a tall with a dwarf, giving attention to this pair
of contrasted characters alone, he found that all the first hybrid
generation were talls, with no dwarfs and no intermediates. Accordingly
he called the tall character _dominant_, and the dwarf character
_recessive_, and a pair of contrasted characters which act in this way
are now called _factors_ or sometimes called _unit characters_. But on
allowing these hybrids to cross-fertilize one another in the usual way,
Mendel found that in the second generation of hybrids there were
_always_ _three talls to one dwarf_ out of every four. Further
experiments proved that these dwarfs of the second hybrid generation
_always bred true_, that is, one out of four; and that one out of the
remaining talls always bred true, making another quarter of the total;
while the remaining fifty per cent. proved to be mixed tails, always
acting as did the original hybrids, splitting up in the next generation
in the same arithmetical proportion as before.

Accordingly, if we confine our study to the two contrasted characters,
tallness and dwarfness, we see that just three kinds of peas exist,
namely, dwarfs which breed true, talls which breed true, and talls which
always give the same definite proportion of talls and dwarfs among their
descendants. Innumerable experiments which have since been made with
other pairs of characters have demonstrated that this same mathematical
proportion holds good throughout the whole world of plants and
animals;[25] and hence this astonishing result is now called Mendel's
Law, and is regarded as the most important discovery in biology in
several generations.