By ALFRED NEWTON.
Some facts as to the geographical distribution—whether of plants
or animals—have, it is true, been long known, indeed they present
themselves on the slightest inquiry. Every one is aware that elephants
and tigers do not roam in our woods now-a-days, whatever may have been
the case aforetime. Many persons have read that horses were unknown
in the New World at the time of its discovery by Europeans, and were
subsequently introduced by its Spanish conquerors. Some may even know
that humming-birds are not to be found in the Old World, and that (as
has been already said) the so-called “marsupial” animals are at the
present time, with a few exceptions, confined to Australia, as well as
that in that country nothing like vultures or woodpeckers are to be
found.
The assemblage of animals which inhabit any portion of the earth’s
surface, whether it be land or water, is called its “fauna,” in the
same way that the plants of a country are called its “flora.” To
be entitled to the former term it is unnecessary that the animals
composing the assemblage should not be found anywhere else; it is
enough that they occur there and impress upon the district, be it
large or small, certain more or less well-marked peculiarities. Nor
does it follow because certain kinds of animals are found to inhabit
two districts that these two have the same fauna. We have to take
the whole assemblage as a whole, and abide by the verdict which the
majority of kinds affords us. Now by collecting such facts as those
stated in the preceding paragraph, and such facts can be collected by
the hundred or the thousand, we are able to get hold of a general idea
of the geographical distribution of animals, and when the results of
all the knowledge on this subject which we can acquire are brought
together, it will appear that the earth may be partitioned into several
great zoölogical regions—each separable in subregions, provinces,
subprovinces and so on.
America is divided into two regions—the “Nearctic” and the
“Neotropical,” which meet in Mexico at about the 22d parallel of north
latitude:—
(1) The Nearctic Region (that is the Northern part of the New World)
includes the Aleutian Islands, besides Greenland and the Bermudas with
all of what is generally called North America.
(2) The Neotropical Region (that is the tropical part of the New World)
comprises the West India Islands, the Galapagos, and the whole of South
and Central America.
Passing to the Old World, it is separable, as may be seen, into four
regions.
(3) The Palæarctic Region (or Northern part of the Old World) including
that portion of Africa which lies to the northward of the Great Desert,
the Atlantic Islands (Madeiras, Canaries, and Azores), the whole of
Europe from Iceland to Greece, besides Asia Minor, Palestine, Persia,
probably Afghanistan, the whole of Northern, Central and Eastern Asia,
lying to the northward of the Himalaya Mountains and of China proper,
as well as Japan.
(4) The Ethiopian Region consists of Africa, excepting Morocco and
Algeria (which, as already stated, belong to the preceding region), as
well as of Arabia and of course the adjacent islands from those off the
Cape Verd to Madagascar and Socotra.
(5) The Indian Region includes possibly Beloochistan, all British
India, Burmah, China proper (that is, without Chinese Tartary), Cochin
China, the Malay Peninsula, Sumatra, Java, Bali, Borneo, and the
Philippine Islands.
(6) The Australian Region is very trenchantly divided from the Indian
at the Straits of Macassar, and, beginning with the islands of Celebes
and Lombok, comprises all the groups between them and Papua or New
Guinea, as well as Australia, Tasmania, New Zealand, and, generally,
all the islands of the Pacific Ocean, except those already otherwise
appropriated—as Japan, the Aleutian Islands and the Galapagos.
It may be added that though the preceding outlines of geographical
distribution were first laid down with reference to the most vagrant
class of animals in creation—namely birds—their truth has since been in
the main confirmed by nearly all those zoölogists who have studied the
subject in reference to particular classes in the knowledge of which
they themselves stand preëminent.
Yet it may not be unreasonably expected of these six zoögeographical
regions, that they are not all equally distinct, and it is quite
possible that future researches may show that their boundaries require
some rectification.
The study of the geographical distribution of animals furnishes us with
facts of much importance in the history of the earth. For example: It
has been stated, and that on the very best authority, that the marine
faunas of the two coasts of the Isthmus of Panama, which joins the
two continents of North and South America, have but thirty per cent.
of species in common. Now what does this show? No doubt the very
considerable antiquity of the barrier which exists between the Atlantic
and the Pacific Oceans—for if, in anything like recent times, there
had been a break in this barrier, within the tropics where the sea is
warm, then assuredly we should have had a very much larger interchange
of the species which inhabit its two sides, or perhaps we should even
find precisely the same fishes, the same shells, the same crabs and the
same corals in the harbor of Colon on the one side and that of Panama
on the other. As it is we have corals on the Atlantic coast of the
isthmus and on the Pacific none whatever, while, as before said, of the
rest of the marine fauna (the fishes especially) not more than thirty
per cent. are common to both. It is moreover particularly to be noted
that there seems to be no other reason than the one here assigned for
this difference. Very many sorts of fishes and of shells which occur on
one side so much resemble those found on the other that the distinction
between them is only such as can be recognized by expert zoölogists,
yet this distinction is constantly to be observed—they form what are
called “Representative Species,” that is, one kind of fish or shell on
one side is exactly represented by another kind of fish or shell on the
other.
But this difference between the marine faunas of the two coasts of the
Isthmus of Panama not only proves its long duration as a barrier of dry
land, but some other deductions follow naturally enough. It is also
tolerably clear that the Gulf Stream must have been running pretty much
the same course that it runs now so long as the barrier presented by
the Isthmus of Panama has existed. If it were not for that barrier the
current would have continued its westerly flow onward to the Pacific
Ocean. Now we have seen that the difference between the marine faunas
of the two sides of the isthmus proves its long duration. Hence we may
fairly conclude that for so long has the Gulf stream been flowing and
helping to soften what would otherwise have been the rigorous climate
of Ireland and Scotland, thereby materially affecting their fauna.
Everyone knows the old legend of St. Patrick, and how he is said to
have banished all noxious reptiles from his favorite island. As a
matter of fact only one kind of reptile proper is found in Ireland.
This is the viviparous lizard, a harmless little animal which also
occurs in Great Britain and generally throughout the continent of
Europe. But in England we have besides a second kind of lizard,
commonly known as the sand-lizard, and this also is spread over the
Continent, where they have in addition, even in Northern France, a
third kind, the green lizard, which does not inhabit any part of Great
Britain or much less of Ireland. It is therefore a not very unlikely
deduction from these facts that the viviparous lizard had made its
appearance in this part of the world at an epoch when Ireland was
joined to England by dry land, and England was in like manner connected
with France, and that that epoch was earlier than the time when the
sand-lizard appeared, for if the latter had then occurred it would
in all likelihood have spread to Ireland. But if we suppose, and
geologists tell us we may do so, St. George’s Channel to have been
formed before the English Channel was, then it is plain that a reptile
extending its range from the middle of Europe would have been able to
get into England, but not into Ireland; and this supposition would
account for the limited distribution of the sand-lizard. While again a
third reptile, like the green lizard, coming at a subsequent period,
after the straits of Dover were formed, would find them before him and
be unable to set his foot off the continent.
Thus in whatever way we regard them, the not unreasonable deductions
afforded by the facts which a study of the geographical distribution
of animals makes known to us are of very great importance. We may of
course be wrong in some of our inferences, we very likely shall err,
as some of our predecessors have done, but the facts remain whatever
construction we put upon them, and, as they go on accumulating, we may
be sure that errors by degrees will be swept away, and perhaps the
genius of man by this means alone may explain one of the mysteries of
creation.