A curious instance of latent characters correlated with sex is seen in hive bees. The worker bee differs from the female in the rudimentary condition of the sexual organs, in size and form, and in the higher development of the sense-organs. But it is well known that, if a very young worker grub be fed on "royal jelly," she will develop into a perfect queen. Not only are the sexual organs stimulated to increased growth and functional activity, but the correlated size and condition of the sense-organs are likewise acquired. The characters of queen and worker are latent in the grub. According to the nature of the food it receives, the one set of characters or the other emerges. Professor Yung's tadpoles and Mrs. Treat's butterflies (_ante_, p. 59) afford similar instances. We come now to those cases of latency in which this obvious correlation does not occur. They afford examples of reversion to more or less remote ancestral characters. In some cases the cause of such reversion--such unexpected emergence of characters, which have remained latent through several, perhaps many, generations--is quite unknown. In others, at any rate among domesticated animals, the determining condition of such reversion is the crossing of distinct breeds. Darwin gives[EC] an instance of reversion, on the authority of Mr. R. Walker. He bought a black bull, the son of a black cow with white legs, white belly, and part of the tail white; and in 1870 a calf, the gr-gr-gr-gr-grandchild of this cow, was born, coloured in the same very peculiar manner, all the intermediate offspring having been black. In man partial reversions are not infrequent. An additional pair of lumbar ribs is sometimes developed, and in such cases the fan-shaped tendons which are normally connected with the transverse processes of the vertebræ are replaced by functional levator muscles. Since it is probable that the ancestor of man had more than the twelve pairs of ribs that are normally present in the human species, we may, perhaps, fairly regard the supernumerary rib as a reversion. But it may be a new sport on old lines. The occasional occurrence in Scotland of red grouse with a large amount of white in the winter plumage, especially on the under parts, is justly regarded by Mr. Wallace[ED] as a good example of reversion or latency in wild birds. There can be little doubt that, as he suggests, the Scotch red grouse is derived from a form which, like the wide-ranging willow grouse, has white winter plumage. During the glacial epoch this would be an advantage. "But when the cold passed away, and our islands became permanently separated from the mainland, with a mild and equable climate, and very little snow in winter, the change to white at that season became hurtful, rendering the birds more conspicuous, instead of serving as a means of concealment." The red grouse has lost its white winter dress; but occasional reversions point to the ancestral habit.