Bonhote has performed a large number of experiments in hybridising ducks. Some of his hybrids were produced from three pure ancestors, as, for example, the pintail, the spotbill, and the mallard; others from two ancestors. Some of these hybrids were crossed with other hybrids, and others with the parent forms, hence Bonhote secured a number of hybrids, each of which had a distinctive appearance; but _all_ the variations appearing among the hybrids were found to start on one or more of the pœcilomeres. Certain of the hybrids showed a resemblance to one or other of the parent species, others were unlike either parent, and resembled either no known species or species other than their parents. When a hybrid shows a resemblance to a species other than that to which either parent belongs, it is said to exhibit the phenomenon of atavism or reversion,—the individual is supposed to have been “thrown back” to an ancestral form. The true explanation of the phenomenon would seem to be that, as the result of the crossing, biological molecules in the fertilised egg have been formed which, on development, give rise to combinations of colour like those seen in other species. Thus the phenomena of “mimicry” and “reversion” are, we believe, due to the fact that in the fertilised egg of both the pattern and its copy a similar arrangement of biological molecules obtains. If we regard the sexual act as resembling in many respects a chemical synthesis, the phenomenon need not surprise us. To sum up, the observed facts of animal colouration seem to indicate that there are in each organism some twelve or thirteen centres of colouring, which we suggest may correspond with portions of the fertilised egg. From each of these centres the colour develops and spreads, so that every part of the organism is eventually coloured. These centres of colouring are not altogether independent of one another. Sometimes they all give rise to the same hue, in which case we have a uniformly-coloured organism, such as the raven. More often from some one colour develops, and from others another colour; if these two colours happen to be black and white, the result is a pied organism, which displays a definite pattern due to the correlation of the various colour-producing biological molecules.