6 
AMERICAN SPECIES OF ARADUS (hEMIPTERA) 
extreme specialization has supervened with its ultimately fatal 
consequences. Examining in detail the criteria which mark the 
various forms, we find that they are relatively few in miinber 
and are such as to indicate that the species have undoubtedly 
arisen liy a "shuffling of characters.” I think, hov'ever, that 
this shuffling must have largely ceased, once the vdable combin- 
ations were hit upon, and that long since, because %ve have to 
do here with true species, each with a characteristic habitus 
resulting from the comliined influence of a multiplicity of minute 
peculiarities in every part of the b.ody, and thus distinguished 
not alone by the few criteria which we find expressible in words. 
Aradus provides us with exceptionally favoralile material for the 
study of the species as a taxonomic unit, because, with this clear 
specific development, there is frequentty a high degree of indi- 
vidual variation. In addition to a certain instability in color 
and size, \vhich is usual among the Hemiptera, there is great 
diversity among individuals in the details of form and even in 
the proportions of parts, especially noticealile in some of the 
species of wide range like prohoscideus and similis, hut usually 
these variations may be recognized as individual peculiarities liy 
their frequent asymmetry, liy intergradation, or bj^ an entire 
lack of correlation with other characters, and there are few which 
are considerable enough to cause doubt in determination or which 
are of such nature as, in my opinion, to reipiire naming. Fur- 
thermore, the permutational type of evolutionary history referred 
to above would tend to produce forms having the relatively 
slight differences in degree of specialization which we meet with 
in this genus — a condition which strikes us at once in the attemiit 
to arrange the sjiecies in an aiiproximately natural linear seipience 
or ('ven in groups. 
Arrangement of the Species in (iRoups 
Fhere are, however, various degrees of relationshi]) among the 
species, which may be indicated by the establishment of groups, 
but it must be borne in mind that these groups ari' by no means 
sharply distinguished, and both the groups and the spi'cies of 
M'hich tlu'y are (‘omposed ('xhibit complicati'd int('rrelationshi])s 
which ar(‘ (piite bi'yond (‘X])r('ssion in a lini'ar ai'rangement. 
