isen
in which the wings are bent upwards and in the most extreme type the wings
are curled over the back, as seen in figure 54 (g), yet there is no
historical connection between these stages.

Mutations have occurred involving the pigmentation of the body and wings.
The head and thorax of the wild Drosophila ampelophila are grayish yellow,
the abdomen is banded with yellow and black, and the wings are gray. There
have appeared in our cultures several kinds of darker types ranging to
almost black flies (fig. 20) and to lighter types that are quite yellow. If
put in line a series may be made from the darkest flies at one end to the
light yellow flies at the other. These types, with the fluctuations that
occur within each type, furnish a complete series of gradations; yet
historically they have arisen independently of each other.

Many changes in eye color have appeared. As many as thirty or more races
differing in eye color are now maintained in our cultures. Some of them are
so similar that they can scarcely be separated from each other. It is
easily possible beginning with the darkest eye color, sepia, which is deep
brown, to pick out a perfectly graded series ending with pure white eyes.
But such a serial arrangement would give a totally false idea of the way
the different types have arisen; and any conclusion based on the existence
of such a series might very well be entirely erroneous, for the fact that
such a series exists bears no relation to the order in which its members
have appeared.

Suppose that evolution "in the open" had taken place in the same way, by
means of _discontinuous_ variation. What value then would the evidence from
comparative anatomy have in so far as it is based on a continuous series of
variants of any organ?

No one familiar with the entire evidence will doubt for a moment that these
125 races of Drosophila ampelophila belong to the same species and have had
a common origin, for while they may differ mainly in one thing they are
extremely alike in a hundred other things, and in the general relation of
the parts to each other.

It is in this sense that the evidence from comparative anatomy can be used
I think as an argument for evolution. It is the resemblances that the
animals or plants in any group have in common that is the basis for such a
conclusion; it is not because we can arrange in a continuous series any
particular variations. In other words, our inference concerning the common