ta males and females and lacticolor females. Reciprocally
lacticolor male by grossulariata female, (fig. 42) gives lacticolor
daughters and grossulariata sons and these inbred give grossulariata males
and females and lacticolor males and females.

[Illustration: FIG. 41. Sex-linked inheritance in the wild moth, Abraxas
grossulariata (darker) and A. lacticolor.]

[Illustration: FIG. 42. Reciprocal of Fig. 41.]

[Illustration: FIG. 43. Four wild types of Paratettix in upper line with
three hybrids below.]

It has been found that there may be even more than two factors that show
Mendelian segregation when brought together in pairs. For example, in the
southern States there are several races of the grouse locust (Paratettix)
that differ from each other markedly in color patterns (fig. 43). When any
two individuals of these races are crossed they give, as Nabours has shown,
in F_2 a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there
are here at least nine characters, any two of which behave as a Mendelian
pair. These races have arisen in nature and differ definitely and
strikingly from each other, yet any two differ by only one factor
difference.

[Illustration: FIG. 44. Diagram illustrating four allelomorphs in mice,
viz. gray bellied gray (wild type) (above, to left); white bellied gray
(above, to right); yellow (below, to right); and black (below, to left).]

Similar relations have been found in a number of domesticated races. In
mice there is a quadruple system represented by the gray house mouse, the
white bellied, the yellow and the black mouse (fig. 44). In rabbits there
is probably a triple system, that includes the albino, the Himalayan, and
the black races. In the silkworm moth there have been described four types
of larvae, distinguished by different color markings, that form a system of
quadruple allelomorphs. In Drosophila there is a quintuple system of
factors in the sex chromosome represented by eye colors, a triple system of
body colors, and a triple system of factors for eye colors in the third
chromosome.

MUTATION AND EVOLUTION

What bearing has the appearance of these new types of Drosophila on the
theory of evolution may be asked. The objection has been raised in fact
that in the breeding work with Drosophila we are dealing with artificial
and unnatural conditions. It has been more than implied that results
obtained from the breeding pen, the seed pan, the flower pot and the milk
b