DOES HYBRIDISATION INCREASE FLUCTUATING VARIABILITY? 103 
have the same value when judged by the offspring’s qualities. Here the 
special resemblance between brothers and sisters has nothing at all to do 
with heredity as defined by the characters of the zygotes or gametes, I 
cannot here enter into this matter, but it was necessary to point out this 
recent biological fault (confusion of social problems with biolocical—as 
always by Pearson) because it is important to demonstrate how this very 
excellent mathematician errs when dealing with biological questions of 
heredity. Nay, heredity can only be studied in an exact manner by 
breeding experiments, and here in two ways—analysis and synthesis. 
The analytical experiment is in its clearest and purest form carried out 
by working with “pure lines,” 7.¢. individuals descending from one single 
homozygotic individual. Pure lines are only to be had in organisms 
with self-fertilisation (or parthenogenesis) ; multiplication by eraftings, 
cuttings and other forms of vegetative propagation can here be left out 
of sight. 
“Pure line” is a mere genealogical term ; different authors have un- 
fortunately misconceived this meaning, and confounded “ pure lines’ with 
“ types,’ “small species,’ and other such things. I must energetically 
protest against this misrepresentation of my term “pure line.” It in- 
dicates nothing more than the warranted purity of descent. By muta- 
tion or segregation new types of gametes can be formed within pure 
lines as well as in genealogical hybrids—the line remains notwithstanding 
as pure as before in the genealogical sense. Pure lines, therefore, can be 
monotypical or bi- and polytypical. When we only have quantitatively 
determined types in view, we may express the fact by the words mono-, 
bi- and polymodal pure lines. Hitherto, I have only published a few 
of my researches in monomodal pure lines. As my work is proceeding 
I hope to publish the results of experiments with bi- and polymodal 
“lines,” the behaviour of which in some points may have resemblance 
with the segregation in genealogical hybrids. Such occurrences having 
been found in pure lines seem to me to have a special and peculiar 
interest, affecting also the cytological problems of heredity. The time at 
my disposal does not allow me to enter into this matter here. 
Experiments with monomodal pure lines have shown me that Galton’s 
law of filial regression (in all those cases where this law has been analysed 
by means of pure lines) is only a consequence of the fact that the popula- 
tions in question contain different types of organisms. And this composite 
character of a population cannot be recognised by inspection or any 
computation of the variations! Selection acts in all such cases apparently 
as a type-displacing factor ; in reality, selection has no altering influence 
as to the nature of the existing biotypes. Selections act only as sorting 
factors, more or less perfectly isolating that type or those types which 
differ most from the average of the population. 
The continued researches which I have carried out during the last 
four years have only confirmed this view, and it will be seen that this is in 
accordance with the practical experiences from Svaléf. I have tried to find 
special cases where an effect of selection could be recognised, but in vain. 
Thus it might be supposed that special selection of the very smallest 
seeds would give weakly plants, the seeds of which in their turn would be 
badly nourished, and therefore small ; but even this reaction (which must 
