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.
In order to ascertain whether no crayfish exist in which the characters of the parts here referred to are intermediate between those defined, it would be necessary to examine numerous examples of each kind of crayfish from all parts of the areas which they respectively inhabit. This has been done to some extent, but by no means thoroughly; and I think that all that can be safely said, at present, is that the existence of intermediate forms is not proven. But, whatever the constancy of the {296} differences between the two kinds of crayfishes, there can surely be no doubt as to their insignificance; and no question that they are no more than such as, judging by analogy, might be produced by variation.
From a morphological point of view, then, it is really impossible to decide the question whether the stone crayfish and the noble crayfish should be regarded as species or as varieties. But, since it will, hereafter, be convenient to have distinct names for the two kinds, I shall speak of them as Astacus torrentium and Astacus nobilis.[20]
[20] According to strict zoological usage the names should be written A. fluviatilis (var. torrentium) and A. fluviatilis (var. nobilis) on the hypothesis that the stone crayfish and the noble crayfish are varieties; and A. torrentium and A. fluviatilis on the hypothesis that they are species; but as I neither wish to prejudge the species question, nor to employ cumbrously long names, I take a third course.
In the physiological sense, a species means, firstly, a group of animals the members of which are capable of completely fertile union with one another, but not with the members of any other group; and, secondly, it means all the descendants of a primitive ancestor or ancestors, supposed to have originated otherwise than by ordinary generation.
It is clear that, even if crayfishes had an unbegotten ancestor, there is no means of knowing whether the stone crayfish and the noble crayfish are descendants of the same, or of different ancestors, so that the second sense of species hardly concerns us. As to the first sense, there is no evidence to show whether the two {297} kinds of crayfish under consideration are capable of fertile union or whether they are sterile. It is said, however, that hybrids or mongrels are not met with in the waters which are inhabited by both kinds, and that the breeding season of the stone crayfish begins earlier than that of the noble crayfish.
M. Carbonnier, who practises crayfish culture on a large scale, gives some interesting facts bearing on this question in the work already cited. He says that, in the streams of France, there are two very distinct kinds of crayfishes—the red-clawed crayfish (L’Écrevisse à pieds rouges), and the white-clawed crayfish (L’Écrevisse à pieds blancs), and that the latter inhabit the swifter streams. In a piece of land converted into a crayfish farm, in which the white-clawed crayfish existed naturally in great abundance, 300,000 red-clawed crayfish were introduced in the course of five years; nevertheless, at the end of this time, no intermediate forms were to be seen, and the “pieds rouges” exhibited a marked superiority in size over the “pieds blancs.” M. Carbonnier, in fact, says that they were nearly twice as big.
On the whole, the facts as at present known, seem to incline rather in favour of the conclusion that A. torrentium and A. nobilis are distinct species; in the sense that transitional forms have not been clearly made out, and that, possibly, they do not interbreed.