100 REPORT OF THE CONFERENCE ON GENETICS. 
Darbishire’s, Guaita’s, Hurst’s, Lang’s and others’ experiments (with 
animals) are so plain and clear just because the characters in question 
are “ qualities.” 
The problem whether the Mendelian segregation is absolutely pure is 
a matter of a special nature, giving no loophole for the biometrical view 
of ancestral influence. By impure segregation, when a small quantity of 
“substance which ought to have been cleaned out” is carried over 
with the gamete, and finds conditions in the zygote for increasing, 
impurity may be increasingly augmented. And at last it will become 
manifest in some individuals in a generation possibly very far 
removed from the ancestor in question. I have made some experiments 
on this point, but this is not the place for discussing such matters more 
closely. 
Now I come to the domain in which the stronghold of biometry is 
situated : the traits which are characterised quantitatively, the types that 
manifest themselves as differences in degree. Here we meet with the 
ereatest difficulties; here we cannot by simple inspection of any indi- 
vidual decide its type. Here we meet with the “ transgressive variability,” 
which makes it quite impossible to judge by inspection whether an 
individual specimen is a plus-variant of a “little”? type or a minus- 
variant of a “large ”’ type, and so on. 
The most important and conspicuous results of the Mendelian experi- 
ments relate to traits that do not blend, and with regard to which every 
simple individual can be grouped in the right class immediately. The 
results are therefore very striking and well fitted for popular demonstration. 
In De Vries’s celebrated studies of mutations it is almost always such 
qualities which are regarded; the same may also be said about the exten- 
sive and important experiments carried out at Sval6f, in Sweden (5). Here 
“botanical’’ characters are almost exclusively regarded, 7.e. unmistakable 
morphological characters, which—as De Vries has said—are “ traits not of 
fluctuating but of mutative nature.” These morphological characters are 
constant except when mutations suddenly give rise to new types. That the 
pure “pedigrees” of Svalof in reality have constant types in respect of 
the quantitatively characterised traits—which give the crops their value—is 
for me a matter of course, and is also asserted by Professor Hj. Nilsson, 
of Syalo6f. But conclusive scientific researches about all such highly 
fluctuating characters have not yet been made at Svalof, where the 
excellent special workers with good reason have taken “ botanical ”’ 
characters as starting-points for their isolation of types. 
The study of heredity as to characters, which by inspection can only 
be estimated as differences in intensity of the same quality, and which 
blend in hybridisation, requires special methods. The hybrids with such 
characters have not yet been examined in a satisfactory manner. In my 
experiments with “ pure lines” (6) I particularly tried to isolate quantita- 
tively different types from the population in question, and in that way I 
—as the first, I believe—found out that the Galtonian law of filial 
regression, declaring that fluctuations are to a certain considerable degree 
hereditary, is quite wrong and only depends on the presence of several 
different types in the populations. Ina population containing only one 
single type the selection of fluctuations has no action at all! The just- 
