DOES HYBRIDISATION INCREASE FLUCTUATING VARIABILITY ? 105 
In the domain of hybridology Mendelian analysis has cleared away 
very much of the obscurity which until recent years was reigning here. 
It has been the easily appreciable qualitatively characterised traits which 
here have -been the objects of research, and hence in cross-fertilising it 
has mostly not been necessary to use individuals of which the type- 
characters in other respects have been determined by special experiments 
in several generations. Perhaps the neglect of this point may have given 
to some series of hybrid descendants a greater heterogeneity than would 
have been encountered by intercrossing individuals belonging to the same 
pure line of one variety with similarly constituted individuals of another 
variety (or species). 
However it may be in this question; when we proceed to researches 
in the hybridisation of types that are quantitatively characterised, the 
highest degree of purity in the two intercrossing varieties or species is 
required. The material for such hybridisation experiments—to be of 
scientific value—must be pure lines, the constancy (or, if it may be, the 
mutability, segregative capacity, and so on) of which has been previously 
studied in a sufficient number of generations. 
‘We here again touch the fundamental problem as to selection and 
continuous variability, but now with the complication of intercrossing. 
Here general scientific opinion sticks to the very popular idea that 
selection—continued again and again—is able to displace the type of the 
organisms in question. As to the qualitatively characterised types, 
Mendelism has shown the inadequacy of selection (17), but as to the 
quantitatively characterised types the conception is still alive that selec- 
tion will be able to displace the types in the same direction as the 
selection is made. < 
Here I may give some remarks about some criticisms of my paper 
-on heredity in pure lines (6). Professor Plate (18) has quite misin- 
terpreted my views. I maintain that in (monomedal) pure lines no effect 
of selection has been proved; I never spoke of an effect which goes 
back when selection is stopped—here Plate has confounded me with 
De Vries, who has not worked with pure lines (19). One of the chief 
points in my little work is that I regard selection of fluctuations as quite 
ineffective, and hence must emphasise an absolute difference between 
fluctuation and mutation—at least as to their perceptible manifestations. 
Here I must see more than “ difference of degree.’ When recently, 
besides the biometricians and Plate, also an eminent experimenter, 
Professor Lang (20), basing his views upon very interesting breeding 
experiments with snails, declares that mutation and fluctuation only are 
different in degree, then we are at a point of irreconcilable opposition. 
We are here concerned with one of the most important fundamental 
problems in heredity—even the very conception of the meaning of 
“ heredity ’’ is affected. This is manifested by such expressions of Lang 
as “ different degrees in the heredity of recurring unaltered characters ”’ 
(‘‘ verschiedene Grade der Erblichkeit unverindert wieder auftauchender 
Merkmale”’), and that heredity may be augmented or diminished in the 
course of generations (“dass sich die Erblichkeit im Verlaufe der 
Generationen steigern oder vermindern kann’’), All these expressions 
recall Vilmorin’s (21) idea as to a greater or smaller hereditary power 
