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 wiU, hereafter, be convenient 

 to have distinct names for the two kinds, I shall speak 

 of them as Astacus torrentium and Astacus ndbilis* 



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 Btriot zoological usage the names should be written 

 A.flvviaMis (var. twrentiurn) and A. Jfuviatilis (rar. noMlis) on the 

 hypothesis that the stone crayfish and the noble crayfish are varieties ; 

 and A. torrentinm. and A.flvviatilis 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. 



