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H. H. BARTLETT 
dence of the prevalence of hybridization? The answer is simple. 
The mutations, as a rule, are closely enough related to their parent 
species so that they hybridize readily with them. By hybridization 
the effect of a single mutation may be widely extended, for there is no 
experimental evidence that mutations can be "swamped out" by 
hybridization if otherwise fitted to survive. In groups which have 
perhaps long since passed their zenith and are now represented by a 
few very unlike species hybridization cannot readily take place be- 
cause the species which remain are too unlike to hybridize. Mutation 
and hybridization are usually associated with one another, and I do 
not see how we can escape the conclusion that hybridization is sub- 
sidiary to mutation rather than mutation to hybridization. Both 
processes are simultaneously concerned in the evolution of such poly- 
morphic genera as Oenothera, Rubus, Crataegus and Viola. 
In the beginning of this paper three aims of the experimental 
geneticist were stated. The first, as we have seen, has been attained 
with fair success. The origin of many spontaneous variations has 
been observed, and their genetic and systematic characters have been 
studied. The second aim was to determine the causes of mutability, 
so as to be able to produce the condition at will. Mutations have 
indeed been produced experimentally in the case of the bivalent moss 
varieties which have already been referred to, but other work along 
this line has been unsuccessful. It is impossible to view as conclusive 
the experiments of MacDougal in which genetic variation is supposed 
to have been induced by the injection of various solutions into the 
ovaries of Oenothera (Raimannia) odorata and a species called '^Oeno- 
thera biennis.'' Since MacDougal's views have had such wide pub- 
licity it may be well to summarize briefly the experimental evidence 
which he has brought forward. One mutation is said to have been 
induced in the strain called ''Oe. biennis." This strain was started 
from presumably unguarded seeds of one individual mother plant, of 
which the first generation progeny were grown in 1904. We are no- 
where given any idea how large this progeny was. Four plants were 
self-pollinated, and the progenies were grown from each in 1905. We 
are told that one progeny included 669 individuals, and that the rest 
were not counted. With this generation of 1905, the first to be grown 
from guarded seed, the injection experiments were made. An ovary 
was injected with i : 500 zinc sulphate solution. The seeds obtained 
gave a progeny, the size of which we are not told, in which the solitary 
