THE FOSSIL LOON, COLYMBOIDES MINUTUS

purely on similarities in structure; but with the acceptance of the concept of organic evolution, the corollary of convergence slowly became understood. This idea that two or more unrelated groups of organisms could, in the course of evolution, become superficially similar in both habits and structure is, of course, very important. Thus in order to understand the true relationship between two groups, common characters which are the result of convergent evolution must be distinguished from more conservative features which are indicators of actual phylogenetic relationships. The diving birds provide excellent examples of convergence. Until the appearance of the fourth edition of the Check-List of North American Birds in 1931, the loons, grebes, and auks were all grouped in one order by most American ornithologists. Members of the auk tribe, which, unlike the loons and grebes, regularly use their wings for propulsion under water, are now believed to be related to the gulls. The loons and grebes have been put in separate orders, which, however, are kept side by side in the classifications of most workers. The reasons for separating loons from grebes are many and fundamental. Among them are the differences in the texture of the plumage, the form of the tail feathers, the location of the nest, the number of eggs, and the webbed versus lobed toes. Stolpe (1935) made detailed anatomical studies and listed a number of perhaps more deepseated characters. For instance, when a swimming loon brings its foot forward prior to making a stroke, the toes are folded together and brought straight forward, whereas a grebe turns the whole foot through an arc of 90' and brings it forward sideways. In grebes, the extension of the tibia over the knee joint is made up of both the tibia itself and the patella. In loons, the patella is minute and does not form part of this cnemial crest. Considering that the wings and pectoral girdle are little, if at all, modified for the birds' aquatic habits, the shape of the sternum, coracoid, and, in fact, all the wing elements of loons and grebes are strikingly different. This evidence that loons and grebes were not derived from a common swimming ancestor, however, was long in being discovered, and its significance was obscured for many years by the description of the fossil known as Colymboides minutus. This bird was found in Aquitanian deposits-late Oligocene or early Miocene in age-of France. Three elements, the femur, the humerus, and the ulna, were described by Milne-Edwards (1867-1868). Writing at a time when the role of convergence in evolution was not well understood, he said of the osteological characters of the family of the "Colymbides" [loons and grebes] "Cette famille, comprenant les Plongeons proprement dits et les Grebes, constitue un groupe parfaitement naturel, dont tous les membres presentent entre eux de grands traits de ressemblance." (p. 278). He described Colymboides minutus as having the characters of both loons and grebes and concluded that it was intermediate between these groups of birds. Since from the description, Colymboides was obviously a diving bird and since it was my conviction that loons and grebes could not have come from a common swimming