We call two groups species, if we know of no transitional forms between them, and if there is no reason to believe that the differences which they present are such as may arise in the ordinary course of variation. But it is impossible to say whether the progress of inquiry into the characters of any group of individuals may prove that what have hitherto been taken for mere varieties are distinct morphological species; or whether, on the contrary, it may prove that what have hitherto been regarded as distinct morphological species are mere varieties. What has happened in the case of the crayfish is this: {294} the older observers lumped all the Western European forms which came under their notice under one species, _Astacus fluviatilis_; noting, more or less distinctly, the stone crayfish and the noble crayfish as races or varieties of that species. Later zoologists, comparing crayfishes together more critically, and finding that the stone crayfish is ordinarily markedly different from the noble crayfish, concluded that there were no transitional forms, and made the former into a distinct species, tacitly assuming that the differential characters are not such as could be produced by variation. It is at present an open question whether further investigation will or will not bear out either of these assumptions. If large series of specimens of both stone crayfishes and noble crayfishes from different localities are carefully examined, they will be found to present great variations in size and colour, in the tuberculation of the carapace and limbs, and in the absolute and relative sizes of the forceps. The most constant characters of the stone crayfish are:— 1. The tapering form of the rostrum and the approximation of the lateral spines to its point; the distance between these spines being about equal to their distance from the apex of the rostrum (fig. 61, A). 2. The development of one or two spines from the ventral margin of the rostrum. 3. The gradual subsidence of the posterior part of {295} the post-orbital ridge, and the absence of spines on its surface. 4. The large relative size of the posterior division of the telson (G). On the contrary, in the noble crayfish:— 1. The sides of the posterior two-thirds of the rostrum are nearly parallel, and the lateral spines are fully a third of the length of the rostrum from its point; the distance between them being much less than their distance from the apex of the rostrum (B). 2. No spine is developed from the ventral margin of the rostrum. 3. The posterior part of the post-orbital ridge is a more or less distinct, sometimes spinous elevation. 4. The posterior division of the telson is smaller relatively to the anterior division (H). I may add that I have found three rudimentary pleurobranchiæ in the noble crayfish, and never more than two in the stone crayfish.