But, in all these respects, individual specimens of _A. nobilis_ vary in the direction of _A. leptodactylus_ and _vice versâ_; and if _A. angulosus_ and _A. pachypus_ are varieties of _A. leptodactylus_, I cannot see why Gerstfeldt’s conclusion that _A. nobilis_ is another variety of {304} the same form need be questioned on morphological grounds. However, Kessler asserts that, in those localities in which _A. leptodactylus_ and _A. nobilis_ live together, no intermediate forms occur, which is presumptive evidence that they do not intermix by breeding. * * * * * No crayfishes are known to inhabit the rivers of the northern Asiatic watershed, such as the Obi, Yenisei, and Lena. None are known[27] in the sea of Aral, or the great rivers Oxus and Jaxartes, which feed that vast lake; nor any in the lakes of Balkash and Baikal. If further exploration verifies this negative fact, it will be not a little remarkable; inasmuch as two[28], if not more, kinds of crayfishes are found in the basin of the great river Amur, which drains a large area of north-eastern Asia, and debouches into the Gulf of Tartary, in about the latitude of York. Japan has one species (_A. japonicus_), perhaps more; but no crayfish has as yet been made known in any part of eastern Asia, south of Amurland. There are certainly none in Hindostan; none are known in Persia, Arabia, or Syria. In Asia Minor the only recorded locality is the Rion. No crayfish has yet been discovered in the whole continent of Africa.[29] {305} [27] It would be hazardous, however, to assume that none exist, especially in the Oxus, which formerly flowed into the Caspian. [28] _A. dauricus_ and _A. Schrenckii_. [29] Whatever the so-called _Astacus capensis_ of the Cape Colony may be, it is certainly not a crayfish. Thus, on the continent of the old world, the crayfishes are restricted to a zone, the southern limit of which coincides with certain great geographical features; on the west, the Mediterranean, with its continuation, the Black Sea; then the range of the Caucasus, followed by the great Asiatic highlands, as far as the Corea on the east. On the north, though there is no such physical boundary, the crayfishes appear to be entirely excluded from the Siberian river basins; while east and west, though a sea-barrier exists, the crayfishes extend beyond it, to reach the British islands and those of Japan.