“Or, again, some species occasionally, but not invariably, show a few white feathers on certain parts of their body, and, when such is the case, it will be found that these white feathers appear on the pœcilomeres. . . . There is hardly a species in which examples of these pœcilomeres may not be found. . . . The Kingfisher (_Alcedo ispida_) shows the various head pœcilomeres very clearly, and as examples of inconspicuous differences on these tracts, the rump of the hen sparrow (_Passer domesticus_) and hen chaffinch (_Fringilla cœlebs_), the malar stripe and dark ear-patch of the hen Yellow Bunting (_Emberiza citrinella_), and the dark ante-orbital patch of the Barn Owl (_Strix flammea_) are familiar examples. And, lastly, as an instance of the class where a few white feathers frequently, but not invariably, appear, the young of the cuckoo (_Cuculus canorus_) forms a good example. “These spots may, however, appear in a transitory manner, as, for instance, where a change of plumage (not necessarily moult) is occurring.” As an instance of this, Bonhote cites the case of a young male Shoveler (_Spatula clypeata_), “in which the metallic colour on the head first showed itself on the post-orbital and auricular pœcilomeres, gradually meeting and joining up across the head with the crown and occipital pœcilomeres, and then finally spreading forwards. And it may be well to note that the joining up of the auricular and post-orbital pœcilomeres formed a metallic patch similar in size and position to that found in the male Teal (_Querquedula crecca_), and, further, in the last stage, when the whole head, except the portion round the beak, was metallic, the markings are similar to those found permanently in the hen Scaup (_Fuligula marila_). “Now, these resemblances taking place in the normal pure-bred wild shoveler, the question of reversion does not come in, and no one would suppose these resemblances due to anything more than transitional variation, and it is the object of this portion of the paper to show that variation in colour follows along definite lines.” Biological Molecules