extent than those do from one another. The gills are, in fact, reduced to seventeen on each side, in consequence

FIG. 63. Cambarus Clarkii, male (nat. size), after Hagen. 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

THE ABSTRACTIONS, SPECIES AND GENUS.

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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.

Again, crayfishes are found in the fresh waters of the Southern hemisphere; and almost the whole of what has been said respecting the structure of the English crayfish applies to these; in other words, their general plan is the same. But, in these southern crayfishes, the podobranchiæ have no distinct lamina, and the first somite of the abdomen is devoid of appendages in both sexes. The southern crayfishes, like those of the Northern hemisphere, are divisible into many species; and these species

are susceptible of being grouped into six genera-Astacoides (fig. 65), Astacopsis, Chæraps, Parastacus (fig. 64),

FIG. 64.-Parastacus brasiliensis (nat. size). From southern Brazil,

Engaus, and Paranephrops-on the same principle as that which has led to the grouping of the Northern forms into two genera. But the same convenience which has

led to the association of groups of similar species into genera, has given rise to the combination of allied genera into higher groups, which are termed Families. It is obvious that the definition of a family, as a statement of the characters in which a certain number of genera agree, is another morphological abstraction, which stands in the same relation to generic, as generic do to specific abstractions. Moreover, the definition of the family is a statement of the plan of all the genera comprised in that family.

The family of the Northern crayfishes is termed Potamobiida; that of the Southern crayfishes, Parastacidae. But these two families have in common all those structural characters which are special to neither; and, carrying out the metaphorical nomenclature of the zoologist a stage further, we may say that the two form a Tribe-the definition of which describes the plan which is common to both families.

It may conduce to intelligibility if these results are put into a graphic form. In fig. 66, A. is a diagram representing the plan of an animal in which all the externally visible parts which are found, more or less modified, in the natural objects which we call individual crayfishes are roughly sketched. It represents the plan of the tribe. B. is a diagram exhibiting such a modification of A. as converts it into the plan common to the whole family of the Parastacidæ. C. stands in the same relation to the Potamobiidae. If the scheme were thoroughly worked out, diagrams representing the peculiarities of