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.
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.
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.