Mutations and Evolution. 
65 
through parallel mutations, in several species of (Enothera : but 
tetraploidy exists in a wide range of wild species, and a doubling 
of the chromosome series is also known to occur under various 
experimental conditions which will be referred to later. The 
forms, such as lata and semigig as, with an unbalanced chromosome 
number, will seldom if ever breed true, and such forms can only 
occupy a somewhat temporary and aberrant place in evolutionary 
descent. But tetraploidy is a condition of evolutionary significance, 
as the cytological study of many genera and families already shows. 
It gave the first hints towards the construction of a phylogeny of 
the chromosomes, and ultimately it is quite certain that such a 
phylogeny must be constructed. 
While the 15-chromosome mutants produce two types of 
gametes, and hence are at least dimorphic in their offspring when 
crossed back with the parent type, the tetraploid forms are also 
peculiar in their inheritance, giving triploid intermediate hybrids 
when crossed back with the parent (diploid) type. Such forms are 
for the most part unstable, and although they occur in nature ( e.g . 
the well known natural hybrid between Drosera rotundifolia and 
D. longifolia) yet their instability and partial sterility prevent their 
giving rise to permanent intermediate links between the original 
diploid species and its tetraploid derivative. Meiotic irregularities 
in these hybrids may lead to the production of a series of plants 
with new chromosome numbers, as is well known in the GEnotheras, 
But seldom will a stable form arise having a balanced chromosome 
equipment. With the exception of tetraploidy, therefore, none of 
the types of visible chromosome change so far demonstrated in the 
Oenotheras are likely to have played a very large role in any 
phylogeny. They are chiefly valuable as a means of furnishing a 
visible demonstration of the occurrence of germinal nuclear changes. 
All the evidence indicates that the great mass of mutations 
originate as new Mendelian characters. Among such changes in the 
CEnotheras are (E. rubricalyx , a dominant, and (E . gigas nanella , 
which in crosses with its parent form gigas behaves as a Mendelian 
recessive. We may consider these two forms as types of Mendelian 
mutants. On the basis of their inheritance as such, we must 
assume that they originate through an alteration in a particular 
locus of one chromosome, a view set forth elsewhere 1 and now 
generally accepted. The history of (E. rubricalyx has been given 
in the work cited, 2 but the salient points may be referred to here. 
1 The Mutation Factor, p. 300. 
a The Mutation Factor, p. 102. 
