296 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES. 
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 cray- 
fish 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.* 
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 
* 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 
