In Germany, the two forms have long been popularly distinguished, the former by the name of “Steinkrebs,” {290} or “stone crayfish,” and the latter by that of “Edelkrebs,” or “noble crayfish.” Milne Edwards, it will be observed, speaks of these two forms of crayfish as “varieties” of the species _Astacus fluviatilis_; but, even as far back as the year 1803 some zoologists began to regard the “stone crayfish” as a distinct species, to which Schrank applied the name of _Astacus torrentium_, while the “noble crayfish” remained in possession of the old denomination, _Astacus fluviatilis_; and, subsequently, various forms of “stone-crayfishes” have been further distinguished as the species _Astacus saxatilis_, _A. tristis_, _A. pallipes_, _A. fontinalis_, &c. On the other hand, Dr. Gerstfeldt,[19] who has devoted especial attention to the question, denies that these are anything more than varieties of one species; but he holds this and Milne Edwards’s “second variety” to be specifically distinct from one another. We thus find ourselves in the presence of three views respecting the English and French crayfishes. 1. They are all varieties of one species—_A. fluviatilis_. 2. There are two species—_A. fluviatilis_, and _A. torrentium_, of which last there are several varieties. 3. There are, at fewest, five or six distinct species. Before adopting the one or the other of these views, it is necessary to form a definite conception of the meaning of the terms “species” and “variety.” {291} [19] “Ueber die Flusskrebse Europas.” Mém. de l’Acad. de St. Petersburg, 1859. The word “species” in Biology has two significations; the one based upon morphological, the other upon physiological considerations. A species, in the strictly morphological sense, is simply an assemblage of individuals which agree with one another, and differ from the rest of the living world in the sum of their morphological characters; that is to say, in the structure and in the development of both sexes. If the sum of these characters in one group is represented by A, and that in another by A + _n_; the two are morphological species, whether _n_ represents an important or an unimportant difference. The great majority of species described in works on Systematic Zoology are merely morphological species. That is to say, one or more specimens of a kind of animal having been obtained, these specimens have been found to differ from any previously known by the character or characters _n_; and this difference constitutes the definition of the new species, and is all we really know about its distinctness.