Mutations and Evolution. 
13B 
to each other, has been prodigious and is an achievement in itself. 
The data for sex-linked characters alone involved the breeding of 
over half a million hies. 
Chromosome No. 1, the X, (see Morgan et al. 1915), contains 
all sex-linked mutations, since this is the sex chromosome. Such 
factors already number over 50, including a series of eye colours, 
bar eye with a reduction in the number of facets, yellow and sable 
body colours, forked bristles, club, miniature and rudimentary wings, 
and several lethal factors which prevent development when present 
in the homozygous condition. Chromosome No. 2 is the longest 
pair and contains such mutation factors as purple eye, curved and 
vestigial wings, and black body. No. 3, the next longest, as shown 
by linkage percentages, contains such eye colours as pink and peach, 
rough eye due to a peculiarity of the facets, ebony, sooty and sepia 
body colours, and beaded wings. The fourth group of factors, more 
recently discovered, and transmitted independently of the others in 
the Mendelian sense, contains only two known factors, bent wings 
and the eyeless condition. It evidently corresponds with the fourth, 
very small, pair of chromosomes. 
It has long been known that crossing over, which may occur 
between the various members of each group, is confined to the 
female. The amount of crossing over is also independent of the 
way in which the factors enter an individual in a cross. Thus if 
they enter from the same parent ( i.e ., in the same chromosome) 
any two factors tend to cross over with a certain frequency, while if 
they enter from different parents they tend with equal frequency to 
cross over so that both will be in the same chromosome in the 
offspring. In the male of Drosophila there is no crossing over 
between the X and Y or between any other pairs of chromosomes. 
The probable reason for this has only recently been discovered 
(see Doncaster 1920, p. 235) by Metz. It apparently depends upon 
the fact that while in the egg there is a stage during maturation 
when the long, thin chromosomes are twisted round each other, 
there is no such stage in the male during the period preceding the 
separation of the chromosome pairs, the chromosomes remaining in 
a fairly condensed state during the whole growth period of these 
nuclei. Once again cytology furnishes the basis for the peculiarities 
in the phenomena of inheritance observed. In some other 
organisms (moths and fowls) crossing over takes place only in the 
male and not in the female, while in Primula , grasshoppers and rats 
it takes place in both sexes. That crossing over involves a break 
n the chromosome at a definite point, and not an indiscriminate 
