The points in which very closely allied species are distinguished from each other may be found in the most diverse features of their organisation. Sometimes specific difference is to be seen in a character which we can believe to be important in the struggle, but at least as often it is some little detail that we cannot but regard as trivial which suffices to differentiate the two species. Even when the diagnostic point is of such a nature that we can imagine it to make a serious difference in the economy we are absolutely at a loss to suggest why this feature should be a necessity to species A and unnecessary to species B its nearest ally. The house sparrow (_Passer domesticus_) is in general structure very like the tree sparrow (_P. montanus_). They differ in small points of colour. For instance _montanus_ has a black patch on the cheek which is absent in _domesticus_. The presence in the one species and the absence in the other are equally definite, and in both cases we are equally unable to suggest any consideration of utility in relation to these features. The two species are distinguished also by a characteristic that may well be supposed to be of great significance. In _domesticus_ the two sexes are strongly differentiated, the cock being more ornate than the hen. On the other hand the two sexes in _montanus_ are alike, and, if we take a standard from _domesticus_, we may fairly say that in _montanus_ the hen has the colouration of the male. It is not unreasonable to suppose that such a distinction may betoken some great difference in physiological economy, but the economical significance of this perhaps important distinction is just as unaccountable as that of the seemingly trivial but equally diagnostic colour-point. I have spoken of the fixed characteristics of the two species. If we turn to a very different feature, their respective liability to albinistic variation, we find ourselves in precisely similar difficulty. _Passer domesticus_ is a species in which individuals more or less pied occur with especial frequency, but in _P. montanus_ such variation is extremely rare if it occurs at all. The writer of the section on Birds in the _Royal Natural History_ (III., 1894-5, p. 393) calls attention to this fact and remarks that in that species he knows no such instance. The two species therefore, apart from any differences that we can suppose to be related to their respective habits, are characterised by small fixed distinctions in colour-markings, by a striking difference in secondary sexual characters, and by a difference in variability. In all these respects we can form no surmise as to any economic reason why the one species should be differentiated in the one way and the other in the other way, and I believe it is mere self-deception which suggests the hope that with fuller knowledge reasons of this nature would be discovered.