THE MUTATION THEORY 363 
“Baur’s third category of variations,” say Babcock and Clausen,' 
“comprises all inheritable changes due to causes other than segregation 
and recombination of genetic factors. Although comparatively little 
is known concerning the specific causes of mutations, yet it is possible 
to distinguish between two general classes of such inheritable varia- 
tions according to the nature of the genetic units involved. These 
classes are (1) alterations in genetic factors, and (2) deviations in the 
number of chromosomes. We designate the first group as factor 
mutations and the second as chromosome aberrations. Since the 
first group is of vastly greater importance to agriculture than the 
second, we shall consider the latter very briefly before engaging in 
discussion of the former, which we deem worthy of recognition as 
mutations in the strict sense. 
“Chromosome aberrations.—By the aid of cytology it has been 
demonstrated that. inheritable changes are occasionally induced, in 
plants at least, by irregularities in the behavior of the chromosomes 
during mitosis or meiosis, such that certain germ cells contain fewer 
or more chromosomes than the number typical of the species. Aber- 
rant forms in several plant families are now known to differ from the 
parent species in chromosome number. Some have only a single 
chromosome more or less than the parent, while a few are known in 
which the original number is doubled. It is possible that aberrations 
occur involving all combinations of numbers between these two 
extremes. In various forms of Lamarck’s evening primrose (Oenothera 
lamarckiana), whose typical number is 14, according to Gates the 
following aberrant numbers have been reported—15, 20, 21, 22, 23, 
27, 28, 29, 30. Aberrations involving the doubling of the number of 
chromosomes typical of the species is known as tetraploidy because 
there are four times the haploid number typical of the parent. Oc- 
casionally aberrations or hybridization between diploid and tetraploid 
forms result in triploidy. 
“There is a limited amount of evidence which indicates that 
groups of species have arisen by progressive alterations in chromosome 
number. Thus in Drosophila, Metz has found ten species in which 
the chromosome numbers range from 6 to 12 and the larger numbers 
appear to have arisen by subdivision of the large dumbbell-shaped 
chromosomes found in the species having smaller numbers. Evidence 
that doubling of the chromosome number may occur during somato- 
genesis has been found by Farmer and Digby in the interesting hybrid 
t From E. B. Babcock and R. E. Clausen, Genetics in Relation to Agriculture 
(copyright 1918). Used by special permission of the publishers, The McGraw- 
Hill Book Company. 
