10 GENERA, SPECIES, VARIETIES. 
pose, are so much alike, that they might be called an order of aquatic 
machines of but one family. 
§ 20. Genera. After family manifestations of ordinal characters, we 
come to the modifications of families themselves, enquiring how many kinds 
of difference (genus, a kind, pl. genera) there are in the birds composing a 
family. The mode of determining genera in a family is precisely like that of 
determining families in an order; it is # again (this time representing fam- 
ily characters) into a varying number or combination of characters of the 
next lower grade, a—f. A genus is the last definite grouping of birds that 
is usually recognized; it may be defined as the ultimate essential modifica- 
tion of structure (ultimate, because there is none lower; essential, because 
trivial features do not constitute a genus; of structure, because mere size, 
color, ete., are only specific characters). In the ship family, the three- 
masted vessel, full-rigged, with square sails, is a genus (ship-proper) ; one 
with square sails on two masts only, and fore-and-aft sails on the mizzen, is 
another genus (bark), and so on. Genera are composed of one or more 
§ 21. Species. The definition of a species has become difficult of late 
years, but for present purposes we may assume that it is any one of the 
constant exponents of a genus, comprehending all the birds that bear to 
each other the relation of parent and offspring; the latter capable of repro- 
ducing ‘ each after its kind’ and maintaining certain characters to an evident 
degree peculiar to itself. Resting, then, upon this, we have little else to 
consider before we reach that most unquestionable fact, an individual bird. 
Species, however, are not absolutely constant ; they vary in size, color, ete., 
within certain limits, under influences not always comprehended as yet, but 
which seem a part of that universal tendency in nature toward the produc- 
tion of essential unity in diversity; the operation of which, if completely 
effective, would level distinctions and abolish difference in sameness. 
§ 22. A Variety is a step in this direction; for, although it may seem 
an opposite step, yet departure from any given point or standard must be 
approach toward some other. <A variety is (generally) distinguished from 
a species by its tendency to revert to its original stock, or, diverging fur- 
ther from that, to approach some other type. The former case is constantly 
being demonstrated, and the latter is probably susceptible of being proven ; 
but in either case, énconstancy is a marked feature of varieties. Varieties 
apparently produced by difference in food, climate, etc., are called local 
races, When restricted to a small area in or around the general distribution 
of the parent stock; geographical races, when more widely separated over 
large areas. A hybrid is a cross between two species, almost always of the 
same natural genus. Hybrids are generally infertile, while crosses between 
mere varieties are capable of reproduction, so that hybridism becomes in 
some measure a test; nevertheless, exceptions are not wanting. 
§ 23. Inrermeprare Groupes. Having arrived at the individual bird, we 
will retrace our steps for a moment, for the student must sooner or later 
learn, that, easy as it seems to theoretically determine the foregoing groups, 
