322 GEOGRAPHICAL DISTRIBUTION 
numerous forms of which we are cognizant. But in South America 
a direct connexion with the northern soil must have been _per- 
petuated, and when Monodelphians appeared in what we now call 
Arctogea they must have made their way further south, and thus 
checked the efflorescence of the older stock, restricting it in the New 
World to the one Order which we now find there. Without 
calling in, as so many are apt to do, the aid of a Glacial Epoch, 
it seems that, granting the general continuity of land between 
North and South America,! the older and weaker Marsupial popula- 
tion would give way before the newer and stronger Placental 
population that had become developed in the North. Part of the 
latter established itself in the South, but of the feebler population 
which it dispossessed only a scant remnant exists. Now we may 
not unfairly suppose that the same kind of process went on as 
regards the Birds. It is justifiable to conceive that at one time the 
whole of America was occupied by the ancestors of those forms 
that we now find chiefly displayed in its southern portion, no incon- 
siderable proportion of which still yearly seek their ancient home, 
migrating northward every spring, and returning at the end of 
summer or towards the fall of the year, their original seat being 
occupied by the higher and comparatively recent forms that con- 
stitute part of the Holarctic Fauna—of whose invasion more must 
presently be said. ‘The consequence of all this is that each of the 
Americas presents a mixed population, puzzling to account for until 
the way in which it has been brought about is perceptible. On the 
hypothesis here given, however, the chief difficulties should dis- 
appear, and no evolutionist will regard it as unlikely, forced, or 
impossible—though we may freely grant that its proof requires 
further evidence, but that evidence is of a kind that the marvellous 
success which has attended paleontological research in North 
America induces one to hope may be forthcoming. 
It has just been stated that the general character of the 
Neotropical Fauna is morphologically low. In regard to the Class 
Aves this is shewn by the presence of an Order of Fatitx, consist- 
ing of 3 species of Ruka, and next by the fact that among the 
Carinate the Region claims all the Zinamidx (TINAMOoU)—the 
DRoM&O0GNATH of Prof. Huxley, which, if we carry out his 
principles, ought to be regarded as the equivalent of an Order in 
the usually-accepted sense—and also a unique, very remarkable, 
and generalized form Opisthocomus (HOACTZIN), which he has satis- 
factorily shewn to be so unlike every other that it can only be 
conveniently classed by itself as the sole representative of HETERO- 
1 There can be hardly any assumption here. The existence of corals on the 
Atlantic side of the Isthmus of Panama and their absence on the Pacific Coast 
shews the antiquity of that bridge between the continents, even though it may 
have been sometimes broken down. 
