The general uniformity of the fresh-water fauna throughout the world has often been remarked. Darwin says: "When first collecting in the fresh waters of Brazil, I well remember feeling much surprise at the similarity of the fresh-water insects, shells, etc., and at the dissimilarity of the surrounding terrestrial beings, compared with those of Britain." This uniformity is well illustrated by many of the smaller Crustacea. In a gathering of Cladocera, Copepoda, and Ostracoda, from Central Africa or from Australia, we find that most of the genera, and even some of the species, are identical with those found in similar situations in this country. It is by no means the case that all the species and genera are thus universally distributed, for there are many, especially among the larger forms, which have a very restricted range; but this does not render less striking the general uniformity of the fauna over very wide areas. When we consider the physical environment of fresh-water animals, it seems at first sight as if this wide distribution were the reverse of what might have been expected, for the area occupied by them is far more discontinuous than in the case of terrestrial or marine animals. The inhabitants of a pond or lake are to a great extent isolated; and although they may spread to other ponds and lakes by way of communicating streams or rivers, where these are not too swiftly flowing and are not interrupted by falls, yet direct passage from one river system to another is rarely possible. Further, since practically the whole of the fresh water on the surface of the globe is constantly flowing, more or less rapidly, towards the sea, the smaller feebly swimming forms tend to be swept down with the current, and ultimately carried to perish in the sea. It follows that only those forms which possess special adaptations for dispersal are able to flourish in fresh water. In many cases, as will be described below, the eggs of the smaller Crustacea can survive being dried up, and in this state they may be blown about by wind or carried to great distances in mud, adhering to the feet of migratory wading birds. Darwin says: "The wide-ranging power of fresh-water productions can, I think, in most cases be explained by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream, within their own countries; and liability to wide dispersal would follow from this capacity as an almost necessary consequence" ("Origin of Species," sixth edition, chapter xiii.). In accordance with this, we find that it is just those groups of Crustacea which show these adaptations for dispersal that are most universally distributed in fresh water. On the other hand, the larger Crustacea, like the Crayfishes and River Crabs, which cannot so easily be transported from one locality to another, have as a rule a more restricted range. These larger forms,