

LOTSY, CURRENT THEORIES OF EVOLUTION. 393 
the important question whether the mere fact of heterozygosity 
excludes the possibility of the obtention of definite proof of the 
existence of mutation. . 
To answer this question we must have a clear understanding of 
what is meant by mutation. 
The mutation-concept of DE Vries is based on an assumption, 
on the assumption that an Organism consists of a large number 
of very small living particles, of mutually independant organoids, 
his pangens, capable of multiplication by fission. 
The conception of variability — derived, as we have seen — 
from a confusion of diversity with variability — was, by DE VRIES, 
carried forward to these pangenes. This had the great advantage 
to do away with all confusion of variability with diversity, the 
question now became a very definite one: are genes, orto put the 
question sharper still, is a single gene variable in such a way that 
its constitution is permanently changed? 
Such a change is conceivable, both in a homozygous and in a 
heterozygous organism, I can even imagine the possibility of 
obtaining proof of it — if the assumption of the existence of such 
genes should prove to have hit the mark — in the case of a 
heterozygous organism. so that one is not justified in denying 
offhand the possibility of proof of mutation of this kind from 
heterozygous beings. 
At the present time however, the existence of living pangenes 
is nothing but a hypothesis; which is by no means generally 
accepted; JOHANNSEN even does not hesitate to- call its assumption 
absurd: 
„Viele Zytologen sind — wie es WEISMANN und andere waren — 
geneiët an „organoide Gebilde zu denken, Gebilde die in den 
Zellstructuren (Chromosomen, Chromatinkernen) localisirt sein sollen. 
Vor dieser Auffassung muss entschieden gewarnt werden. Die Genen 
als selbständige „lebende” Elemente auf zu fassen wäre ganz 
ungereimt, auch die Annahme einer Selbständigkeit in dem Sinne, 
dass ein Gen „für sich” im Stande wäre, irgend etwas zu realisiren 
ist bis jetzt unbegründet und fusst offenbar nur in morphologischen 
Auffassungen einer organoiden Natur dieser Einheiten. Der Gesammt- 
Genotypus bestimmt die Erscheinungen von welchen hier die 
Rede ist.” 
