THE GENERAL NAME, SPECIES. 243 



notice agree. No one of these individual crayfishes was 

 exactly like the other ; and to give an account of any 

 single crayfish as it existed in nature, its special peculiari- 

 ties must be added to the list of characters given above ; 

 which, considered together with the facts of structure 

 discussed in previous chapters, constitutes a definition, 

 or diagnosis, of the English kind, or sjjecies, of crayfish. 

 It follows that the species, regarded as the sura of tlie 

 morphological characters in question and nothing else, 

 does not exist in nature ; but that it is an abstraction, 

 obtained by separating the structural characters in which 

 the actual existences— the individual crayfishes — agree, 

 from those in which they differ,, and neglecting the latter. 

 A diagram, embodying the totality of the structural 

 characters thus determined by observation to be common 

 to all our crayfishes, might be constructed ; and it 

 would be a picture of nothing which ever existed in 

 nature ; though it would serve as a very complete 

 plan of the structure of all the crayfishes which are to 

 be found in this country. Tlie morphological definition 

 of a species is, in fact, nothing but a description of the 

 plan of structure which characterises all the individuals 

 of that species. 



California is separated from these islands by a third of the 



circumference of the globe, one-half of the interval being 



I occupied by the broad North Atlantic ocean. The fresh 



waters of California, however, contain crayfishes which are 



