41(1059): 344 (Feb. 13, 1890)
T. D. A. Cockerell
West Cliff, Colorado, U.S.A., January 23.
IN NATURE of November 28, 1889 (p. 80), Prof. G. J. Romanes speaks of galls as ''unequivocal evidence of a structure occurring in one species for the exclusive benefit of another," and states that "it is obvious that natural selection cannot operate upon the plants directly." Nevertheless, there is one way in which galls may be supposed to have been evolved as beneficial—or rather, less harmful—to the plants. Every farmer is aware of the great loss to vegetation caused annually by larvae of insects boring within the branches and twigs of trees. Now suppose that all internal plant feeders were originally borers or leaf-miners—and this is highly probable,—but that some had a tendency to cause swellings in which they fed. These latter would be less injurious to the plants, and the greater the vitality of the plants the more nourishment for them ; and so by degrees the globular and other highly specialized and least harmful galls would be developed, by natural selection, for the benefit not only of the insect, but also of the plant. And known galls, which I need not here enumerate, furnish us with all the steps of this evolution.