INTRODUCTION. 
16 
truly piscivorous birds, as the Loons, Auks, and Cormorants, 
are also placed by themselves, and the series is completed by 
the Gulls, Terns, Fulmars, and other wandering sea-birds, 
which feed on fishes chiefly, but also on Crustacea, insects, 
mollusca, and even birds and mammalia. 
Between the Terrestrial and Aquatic Birds is placed an 
extended series, of which some species are closely allied to 
the one, and some to the other, of these groups. Among 
these “ Grallatores” or “ Waders,” as they are named by 
authors, there are, in fact, some which are not at all addict- 
ed to wading, and others which are almost or entirely as 
much addicted to swimming as the web-footed birds them- 
selves ; so that, had they not, in their aspect and organiza- 
tion, a certain evident connexion, the group might be bro- 
ken up, and its members allotted to the two groups of Land- 
Birds and Water-Birds. In truth, the question is not much 
different from that so much agitated among the geologists, 
whether it be expedient to keep up a Transition class, be- 
tween the Primary and Secondary series, or to separate the 
rocks of that class, referring some to the former, and others ; 
to the latter. The truth is, there is no real break, no line 
of demarcation between the Land-Birds and the Waders, or 
between the latter and the Swimmers. 
Birds have, however, by many authors, been primarily ar- 
ranged into two series : Terrestrial and Aquatic. The former 
may be subdivided into two groups: Yolatorial, or those 
which obtain their prey by flying, and Gradatorial, or such 
as procure it while walking. The aquatic series may, in like 
manner, be divided into two groups : Grallatorial, or long- 
legged birds, addicted to wading, and Natatorial, or birds 
that habitually swim. But, as among the Yolatorial birds 
there may be some that walk occasionally, and among the 
Gradatorial some that are aquatic, although their affinities 
shew that they cannot with propriety be elsewhere placed, 
so among the Waders there are many that seldom, and some 
that never, enter the water, and among the Natatorial some" 
