1918] DEVRIES—MASS MUTATIONS 415 
than the species itself. We further assume a close analogy with 
the mutant hybrids of O. gigas nanella. This conduces to the 
expectation of three types in every generation, namely, constant 
ochracea, constant grandiflora, and hybrid mutants. 
The ochracea are our secondary mutants; they were seen to 
arise in my cultures constantly during the three generations of my 
experiments, and every time in large numbers. The mutant 
hybrids are the apparently normal specimens of grandiflora of my 
strain; they repeat the splitting in every generation, but no con- 
stant grandiflora have been found, since all the specimens tried 
reproduced the mass mutation. Here, therefore, we have to intro- 
duce another principle. This is the assumption of a lethal factor. 
Morcan and his students have discovered the presence of four such 
units in their experiments with Drosophila, and from their studies 
we know exactly what to expect from them (11). I have already 
proposed this principle for the explanation of the empty seeds of 
O. Lamarckiana (4), and we may apply it here in the same way. 
I determined the amount of barren grains among the seeds of my 
strain of O. grandiflora and found 12-41 per cent, with an average 
of 25 per cent for the harvest of 8 self-fertilized plants of different 
generations (4, table on p. 245). Now our argument led us to 
expect 25 per cent of constant specimens, and the hypothesis that 
these are killed within the seeds by some lethal factor would at once 
explain their absence and the presence of the barren grains. 
By means of this hypothesis the conception of our strain of 
O. grandiflora as a hybrid mutant now becomes complete. It 
starts from two succeeding initial mutations in sexual cells, which 
copulated with normal ones. One of these was the mutation into 
a weak, yellowish ochracea, the other was the production of a lethal 
factor, linked to the non-mutated gametes. This linkage must be 
assumed to be so complete as not to interfere with the applicability 
of Mendel’s formula for monohybrids. 
The presence of the lethal factor in both the gametes of a copu- 
lation kills the germ after some time, but the presence of the same 
factor in only one of the two gametes leaves them viable. This 
latter proposition is proved by numerous crosses between species 
with barren grains and those without the factor in question. Such 
seeds are always capable of normal development. 
