10 GENERA, SPECIES, VAEIETIES. 



pose, are so much alike, that they might be called an order of aquatic 

 machiues of but one familj^ 



§ 20. Genera. After family manifestations of ordinal characters, we 

 come to the modifications of families themselves,, enquiring how many hinds 

 of difference {genus, a kind, pi. 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 a; again (this time representing fara- 

 il}' 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 {idtimate, because there is none lower ; essential, because 

 trivial features do not constitute a genus ; of structure, because mere size, 

 color, etc., are only specific chai*acters). 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 difficLdt 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, etc., 

 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 difi'erence 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 b}' its tendenc}' 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, inconstancy 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 widel}' separated over 

 large areas. A hybrid is a cross between two species, almost always of the 

 same natural genus. Hybrids are general!}'' 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. Intermediate Groups. 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, 



