216 MENDELISM 
How far the influence of the Mendelian principles 
may extend we do not yet know. But it is certain that 
very few, if any, cases have so far been discovered in 
which differentiating characters do not behave in this 
way when the types which exhibit them are crossed 
together. Experiments have now been made upon a 
great variety of plants and animals, involving a con- 
siderable diversity of kinds of characters. Nevertheless 
it is scarcely possible to cite a case in which it is 
definitely and certainly known that Mendel’s law, 
subject to the modifications already described, does 
not hold good. Cases of various kinds are, indeed, 
recorded, but these records are derived from experi- 
ments either carried out before the bearings of the 
Mendelian phenomena were at all fully appreciated, or 
—and this is the most frequent case—without any 
knowledge at all of Mendej’s discovery. 
Thus a considerable number of cases were formerly 
described in which the first cross or heterozygote of F, 
bred true instead of segregating in F,. There is some 
doubt whether any case of this kind will really stand 
criticism; Millardet’s case, for example, which was 
described at the end of the last chapter but one, has 
never been confirmed. It is quite certain that among 
all the numerous crosses studied during the last six 
years no example of the kind has been substantiated. 
The most recent cases to be described of a first cross 
breeding true are those of de Vries, and at these we 
are bound to pause, because de Vries is surpassed by 
no recent observer in weight of authority. Neverthe- 
less, de Vries’ cases are of so complex a kind that we 
