We have adduced abundant evidence to show that mutations or discontinuous variations occur in nature; and as these afford much more favourable material on which natural selection can act, it is reasonable to suppose that they have played a considerable part in evolution. When discussing the phenomena of inheritance, we attempted to show that, not improbably, these discontinuous variations are due to some re-arrangement in the constituent parts of the unit characters, or biological molecules, as we have called them. Cranes In this connection we may mention the apparently singular phenomenon of different species in the same natural group, exhibiting either a definite excess or deficiency of plumage on the head. Among cranes, most species are more or less bald; but the Demoiselle (_Anthropoides virgo_) has a fully-feathered head with long side-plumes, while the head of the Stanley Crane (_A. paradisea_) appears to be swollen, so abundantly is it feathered. The crowned cranes, although bare-cheeked, have double crests, the two parts of which have been respectively compared to a pen-wiper and a bunch of toothpicks! Among the guinea-fowls, several species are crested, while others, as, for example, the domestic one, are bare-headed. Now, on the theory of evolution, by accumulation of minute variations, phenomena such as these are difficult of explanation; but, on the assumption that a slight rearrangement of the biological atoms in the molecule may produce very diverse results, as we see in the case of chemical molecules, and of seasonally dimorphic butterflies, there is no particular ground for surprise at such a phenomenon. In this connection we may cite the significant fact, so well known to canary breeders, that two crested birds when mated tend to produce a bald-headed one. If the colour of any part of an organism be due to the internal arrangement of the constituent parts of the biological molecule from which it is derived, we should expect any rearrangement of the component parts to produce quite a different colour. In other words, we should expect occasionally to see colour-mutations. These are precisely what we do see. Similarly, if the scheme of colouring of an organism be due to a certain grouping of biological molecules, we should expect the same scheme of colouring to occur in organisms which are not nearly related. This, too, we observe in nature. Many of the phenomena of mimicry, and all the cases which we have cited as pseudo-mimicry, seem to us to be referable to this. Magpie Colouring Take, for example, the magpie colouration in birds—that is to say, a scheme of colouring in which the body is white, and head, wings, and tail black. This occurs in the following birds belonging to the most diverse groups:— The Magpie. The Magpie Tanager (_Cissopis leveriana_).