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 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; 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. 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 
circumference of the globe, one-half of the interval being 
occupied by the broad North Atlantic ocean. The fresh 
waters of California, however, contain crayfishes which are 
B2 
