322 GEOGRAPHICAL DISTRIBUTION 



numerous forms of which we are cognizant. But in South America 

 a direct connexion mth the northern soil must haA-e been per- 

 petuated, and when Monodelphians appeared in what we now call 

 Ardogsea 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 contin^Hfy^^ \mi^ 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 palseontological 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 

 Ave& this is shewn by the presence of an Order of Ratitx, consist- 

 ing of 3 species of Rhea, and next by the fact that among the 

 Carinatx the Region claims all the Tinamidse (TiNAMOu)— the 

 Drom^ognath^ 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- 



^ There can be hardly auy 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. 



