1921] _ CURRENT LITERATURE 179 
knowledge of the characteristics of mutation are rapidly becoming more 
accurate. Mutations were first ‘discovered’ by Dr Vries in O6enothera 
Lamarckiana, and characterized as being qualitative, discontinuous, and con- 
stant changes in the germ plasm. These three fundamental characteristics 
still hold true, but some of DE Vrre’’ other ideas on the subject have been 
considerably qualified by later work. For convenience the phenomena may 
tentatively be classified under five heads. Information has come largely from 
the published reports of a number of investigators, as indicated, and to some 
degree is supplemented by papers delivered at the last meetings of the Ameri- 
can Association. 
1. Locus CHANGE.—These are changes restricted to a single locus of one 
of the chromosomes. Usually they ate saecuve only on one ey rapeseed of a 
pair, without affecting the on- 
sequently the change first appears in the heterozygous condition. Baur3 
“ce ”) 
n 
mozy 
recessive to the previous condition. Only a very few dominant or “gai 
the basis for progressive evolution. In the fruit-fly these changes take place 
late in gametogenesis, since only one new individual of the new type appears 
in a progeny. Baur, working on Antirrhinum, concludes that changes of 
this sort take place more fccmeitie in the vegetative tissues than in connection 
with gametogenesis, which should result in large numbers of the new type 
among the progeny. In this respect it is quite probable that we are dealing 
with fundamentally different situations in animals, where the germ cells are 
differentiated so early in ontogeny, and in plants, where the germ cells are 
merely late products of permanent growing points. 
LENY‘ states that there is no periodicity to these mutations, thus 
refuting one of the early ideas of De Vries. The same investigator demon- 
strates that reverse mutations are more frequent than original mutations. 
This, however, is simply because they are in the reverse direction, and not 
because of their recent origin. In the case of these reverse mutations, the _ 
changes are always full jumps back to the original starting point, and never 
result in an intermediate condition; nor will the selection of extreme types at 
all modify the rate at which these reverse mutations occur. In the opinion 
of the reviewer, however, this does not completely dispose of the possibility of 
modifying the rate of mutation by selection (it seems quite possible that rate 
of mutation might fall under the influence of multiple modifying factors). 
3 Baur, Erwin, Mutationen von Antirrhinum majus. Zeit. Induct. Abstamm. 
Vererb. 19:177-1093. figs. 10. 1918. 
4 ZELENY, CHARLES, The direction and ireqhency of mutation in a series of 
multiple alleiomorphs. Anat. Rec. 20:210-211. 1921. 
