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 he added to the list of characters given ahove ; 

 which, considered together with the facts of structure 

 discussed in previous chapters, constitutes a definition, 

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

 It follows that the species, regarded as the sum of the 

 morphological characters in question and nothing else, 

 does not exist in nature ; hut 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. The 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 bya third of the 

 jii cumference of the globe, one-half of the interval being 

 u ecu pied by the broad North Atlantic ocean. The fresh 

 waters of California, however, contain crayfishes which are 



