INTERMEDIATE AXD ABERRANT GROUPS. 11 



there lu-e many difficulties m the way of their practical definition. This 

 is partly because all birds are singularly inter-related, presenting few broad, 

 unequivocal, unexceptional characters in the midst of numljcrless minor 

 modifications, and partly because the higher groups, no less than species and 

 varieties, shade into each other. In our illustration, f(n' example, we find 

 exactly intermediate aquatic machines ; thus, it would be difficult for a 

 landsman to say whether an hermaphrodite brig belonged to the ship family, 

 or the schooner family ; he would have to decide according as he considered 

 number of masts, or shape of sails, the more essential family character. 

 But the intermediate groupfi which remain to be examined are not of this 

 ambiguous nature ; they are unequivocally referable to some particular group 

 of the next higher grade, and, Ijeing subordinate divisions, they are distin- 

 guished Ijy the prefix sub, as sub-order, su1)-family. Though somewhat 

 difficult to define, they are, I think, susceptible of intelligible, if not always 

 precise, definition. A sub-group of any grade is framed, without taking 

 into consideration any new or additional characters, upon the varying prom- 

 inence of one or more of the characters just used to form the group next 

 above. In our formula aliove x {abc) for a certain family of the order x, 

 suppose the family character a to bo empJiasized, as it were, and to pre- 

 dominate over b and c, to the partial suppression of these last : then a sub- 

 family of X (abc) might be expressed thus: — x (Abe) ; and it is further 

 evident, that tiiere will be as many sub-families as there are groups of birds 

 in the family representing varying emphasis of a, or b, or c; as x (a B c), x 

 (Ab C), etc. While we take account of 7iew characters of another grade, 

 in forming our successive main groups, in our sub-groups, then, we recog- 

 nize only more or less of the same characters. But the distinction is not 

 always evident; nor is it observed so often as, perhaps, it should be. 



§ 24. TrpiCAL AND Aberrant Groups. Waiving what might be rea- 

 sonably argued against considering any group specially "typical" of the 

 next higher, we may define a convenient and frequent term : — The typical 

 genus of a family, or family of an order, is that one which develops most 

 strongly, or displays most clearly, the more essential characters of the next 

 higher group, of which it is one member. And in proportion as it fails to 

 express these in the most marked manner, either l)y bearing their stamp more 

 lightly, or by having it obscured or defaced by admixture of the characters 

 of a neighboring group, does it become less and less typical (" subtypical ") 

 and finally aberrant. Suppose the ordinal symbol x, as before, to represent 

 the sum of various ordinal characters, more or less essential to the integrity 

 of the order : then obviously, the family characters abc, or def may be com- 

 bined with a varying value of x; thus, x^ (abc) or y^ (def) and the formula 

 of the typical family would Ije x" (a—f). Thus, it is characteristic of most 

 thrushes (Turdidce) to have the tarsus booted, but all do not have it so; 

 therefore, in subdividing the family, we properly make a division into 

 thrushes with l)ooted tarsi, and thrushes with scutellated tarsi ; the former 

 are typical of the family, the latter sub-typical or even aberrant. 



