evolution of the horns of deer with the evolution of worker bees.
Mr. W.I.T. Cunninghame, in his English translation of Eimer's Organic Evolution, says, "No other mammals have been stated to possess two little symmetrical excrescences on the frontal bones as an occasional variation. What caused such excrescences to appear in the ancestors of horned ruminants? Butting would produce them, and no other cause can be suggested which would." The last sentence may be amplified as follows. "Butting would so irritate or stimulate the periostial cells of the frontal bone as to cause them to deposit more bone, thus giving rise to bony excrescences which, transmitted to descendants, and increased during many generations by more butting, would at last result in horns; thus, and thus only, can we account for the evolution of horns." Mr. Cunninghame notwithstanding, it seems to me easy enough to account for the evolution of horns by the theory that inborn variations are alone transmitted, but in doing so we must not seek, as he imagines is necessary, for instances of other mammals which "possess two little symmetrical excrescences on the frontal bones as an occasional variation." Such excrescences would be rare and abnormal, and evolution does not proceed on lines of rare and abnormal variations, which are soon swamped by interbreeding, but on lines of normal variations, as regards which every individual of the species rises above or falls below the specific mean. All ruminants have frontal bones; when butting became of importance in the unconscious struggle for descendants waged by the hornless ancestors of horned ruminants, it is surely clear that those individuals whose skulls were best adapted to butting, who had varied so that their frontal bones were thick and solid, were, other things equal, at an advantage, and were able to kill or drive away from the females their
