estive system; in
the heart and blood vessels; in the muscles.

Each of these systems is very complex, but the same general arrangement is
found in all. Anyone familiar with the evidence will, I think, probably
reach the conclusion either that these animals have been created on some
preconceived plan, or else that they have some other bond that unites them;
for we find it difficult to believe that such complex, yet similar things
could have arisen independently. But we try to convince our students of the
truth of the theory of evolution not so much by calling their attention to
this relation as by tracing each organ from a simple to a complex
structure.

I have never known such a course to fail in its intention. In fact, I know
that the student often becomes so thoroughly convinced that he resents any
such attempt as that which I am about to make to point out that the
evidence for his conviction is not above criticism.

[Illustration: FIG. 4. Drosophila ampelophila. a, Female and b, male.]

Because we can often arrange the series of structures in a line extending
from the very simple to the more complex, we are apt to become unduly
impressed by this fact and conclude that if we found the complete series we
should find all the intermediate steps and that they have arisen in the
order of their complexity. This conclusion is not necessarily correct. Let
me give some examples that have come under my own observation. We have bred
for five years the wild fruit fly Drosophila ampelophila (fig. 4) and we
have found over a hundred and twenty-five new types that breed true. Each
has arisen independently and suddenly. Every part of the body has been
affected by one or another of these mutations. For instance many different
kinds of changes have taken place in the wings and several of these involve
the size of the wings. If we arrange the latter arbitrarily in the order of
their size there will be an almost complete series beginning with the
normal wings and ending with those of apterous flies. Several of these
types are represented in figure 5. The order in which these mutations
occurred bears no relation to their size; each originated independently
from the wild type.

[Illustration: FIG. 5. Mutants of Drosophila ampelophila arranged in order
of size of wings: (a) cut; (b) beaded; (c) stumpy; (d) another individual
of stumpy; (f) vestigial (g) apterous.]

The wings of the wild fly are straight (fig. 4). Several types have ar