If, leaving California, we cross the Rocky Mountains and enter the eastern States of the North American Union, many sorts of crayfishes, which would at once be recognised as such by any English visitor, will be found to be abundant. But on careful examination it will be discovered that all of these differ, both from the English crayfish, and from _Astacus nigrescens_, to a much greater {248} extent than those do from one another. The gills are, in fact, reduced to seventeen on each side, in consequence of the absence of the pleuro-branchia of the last thoracic somite; and there are some other differences to which it is not needful to refer at present. It is convenient to {249} distinguish these seventeen-gilled crayfishes, as a whole, from the eighteen-gilled species; and this is effected by changing the generic name. They are no longer called _Astacus_, but _Cambarus_ (fig. 63). All the individual crayfish referred to thus far, therefore, have been sorted out, first into the groups termed _species_; and then these species have been further sorted into two divisions, termed _genera_. Each genus is an abstraction, formed by summing up the common characters of the species which it includes, just as each species is an abstraction, composed of the common characters of the individuals which belong to it; and the one has no more existence in nature than the other. The definition of the genus is simply a statement of the plan of structure which is common to all the species included under that genus; just as the definition of the species is a statement of the common plan of structure which runs throughout the individuals which compose the species. [Illustration: FIG. 64.—_Parastacus brasiliensis_ (1/2 nat. size). From southern Brazil.]