Volume I Part 10 (1/2)
There is a total absence of all indigenous mammals, reptiles, fresh-water fish, and true land-birds. There is, however, a species of plover, allied to one in South Africa; but it is specifically distinct, and therefore peculiar to the island. The insect life, on the other hand, is abundant. Of beetles no less than 129 species are believed to be aboriginal, and, with one single exception, the whole number are peculiar to the island. ”But in addition to this large amount of specific peculiarity (perhaps unequalled anywhere else in the world), the beetles of this island are remarkable for their generic isolation, and for the altogether exceptional proportion in which the great divisions of the order are represented. The species belong to 39 genera, of which no less than 25 are peculiar to the island; and many of these are such isolated forms that it is impossible to find their allies in any particular country[24].” More than two-thirds of all the species belong to the group of weevils--a circ.u.mstance which serves to explain the great wealth of beetle-population, the weevils being beetles which live in wood, and St. Helena having been originally a densely wooded island. This circ.u.mstance is also in accordance with the view that the peculiar insect fauna has been in large part evolved from ancestors which reached the island by means of floating timber; for, of course, no explanation can be suggested why special creation of this highly peculiar insect fauna should have run so disproportionately into the production of weevils. About two-thirds of the whole number of beetles, or over 80 species, show no close affinity with any existing insects, while the remaining third have some relations, though often very remote, with European and African forms. That this high degree of peculiarity is due to high antiquity is further indicated, according to our theory, by the large number of species which some of the types comprise. Thus, the 54 species of _Cossonidae_ may be referred to three types; the 11 species of _Bembidium_ form a group by themselves; and the _Heteromera_ form two groups. ”Now, each of these types may well be descended from a single species, which originally reached the island from some other land; and the great variety of generic and specific forms into which some of them have diverged is an indication, and to some extent a measure, of the remoteness of their origin[25].” But, on the counter-supposition that all these 128 peculiar species were separately created to occupy this particular island, it is surely unaccountable that they should thus present such an arborescence of natural affinities amongst themselves.
[24] Wallace, _Island Life_, p. 287.
[25] Wallace, _Island Life_, p. 287.
Pa.s.sing over the rest of the insect fauna, which has not yet been sufficiently worked out, we next find that there are only 20 species of indigenous land-sh.e.l.ls--which is not surprising when we remember by what enormous reaches of ocean the island is surrounded. Of these 20 species no less than 13 have become extinct, three are allied to European species, while the rest are so highly peculiar as to have no near allies in any other part of the globe. So that the land-sh.e.l.ls tell exactly the same story as the insects.
Lastly, the plants likewise tell the same story. The truly indigenous flowering plants are about 50 in number, besides 26 ferns. Forty of the former and ten of the latter are peculiar to the island, and, as Sir Joseph Hooker tells us, ”cannot be regarded as very close specific allies of any other plants at all” Seventeen of them belong to peculiar genera, and the others all differ so markedly as species from their congeners, that not one comes under the category of being an insular form of a continental species. So that with respect to its plants no less than with respect to its animals, we find that the island of St.
Helena const.i.tutes a little world of unique species, allied among themselves, but diverging so much from all other known forms that in many cases they const.i.tute unique genera.
_Sandwich Islands._--These are an extensive group of islands, larger than any we have hitherto considered--the largest of the group being about the size of Devons.h.i.+re. The entire archipelago is volcanic, with mountains rising to a height of nearly 14,000 feet. The group is situated in the middle of the North Pacific, at a distance of considerably over 2,000 miles from any other land, and surrounded by enormous ocean depths. The only terrestrial vertebrata are two lizards, one of which const.i.tutes a peculiar genus. There are 24 aquatic birds, five of which are peculiar; four birds of prey, two of which are peculiar; and 16 land-birds, all of which are peculiar. Moreover, these 16 land-birds const.i.tute no less than 10 peculiar genera, and even one peculiar family of five genera. This is an amount of peculiarity far exceeding that of any other islands, and, of course, corresponds with the great isolation of this archipelago. The only other animals which have here been carefully studied are the land-sh.e.l.ls, and these tell the same story as the birds. For there are no less than 400 species which are all, without any exception, peculiar; while about three-quarters of them go to const.i.tute peculiar genera. Again, of the plants, 620 species are believed to be endemic; and of these 377 are peculiar, yielding no less than 39 peculiar genera.
Prejudice apart, I think we must all now agree that it is needless to continue further this line of proof. I have chosen the smallest and most isolated islands for the purposes of our present argument, first because these furnish the most crucial kind of test, and next because they best admit of being dealt with in a short s.p.a.ce. But, if necessary, a vast amount of additional material could be furnished, not only from other small oceanic islands, but still more from the largest islands of the world, such as Australia and New Zealand. However, after the detailed inventories which have now been given in the case of some of the smaller islands most remote from mainlands, we may well be prepared to accept it as a general law, that _wherever_ there is evidence of land-areas having been for a long time separated from other land-areas, there we meet with a more or less extraordinary profusion of unique species, often running up into unique genera. And, in point of fact, so far as naturalists have hitherto been able to ascertain, _there is no exception to this general law in any region of the globe_. Moreover, there is everywhere a constant correlation between the _degree_ of this peculiarity on the part of the fauna and flora, and the _time_ during which they have been isolated. Thus, for instance, among the islands which I have called into evidence, those that are at once the most isolated and give independent proofs of the highest antiquity, are the Galapagos Islands, the Sandwich Islands, and St. Helena. Now, if we apply the method of tabular a.n.a.lysis to these three cases, we obtain the following most astonis.h.i.+ng results. For the sake of simplicity I will omit the enumeration of peculiar genera, and confine attention to peculiar species. Moreover, I will consider only terrestrial animals; for, as we have already seen, aquatic animals are so much more likely to reach oceanic islands that they do not furnish nearly so fair a test of the evolutionary hypothesis.
PECULIAR SPECIES.
+------------+---------+----------+-----------+----------+----------+
Sh.e.l.ls.
Insects.
Reptiles.
Birds.
Mammals.
+---------+----------+-----------+----------+----------+
Sandwich.
400
?
2
16
0
Galapagos
15
35
10
30
0
St. Helena
20
128
0
1
0
+------------+---------+----------+-----------+----------+----------+
Totals.
435
163
12
47
0
+------------+---------+----------+-----------+----------+----------+
NON-PECULIAR SPECIES.
+------------+---------+----------+-----------+----------+----------+
Sh.e.l.ls.
Insects.
Reptiles.
Birds.
Mammals.
+---------+----------+-----------+----------+----------+
Sandwich.
0
?
0
0
0
Galapagos
?
?
0
1
0
St. Helena
0
?
0
0
0
+------------+---------+----------+-----------+----------+----------+
Totals.
0
?
0
1
0
+------------+---------+----------+-----------+----------+----------+
From this synopsis we perceive that out of a total of 658 species of terrestrial animals known to inhabit these three oceanic territories, all are peculiar, with the exception of a single land-bird which is found in the Galapagos Islands. This is the rice-bird, so very abundant on the American continent that its representatives must not unfrequently become the involuntary colonists of the Archipelago. There are, however, a few species of non-peculiar insects inhabiting the Sandwich and Galapagos Islands, the exact number of which is doubtful, and on this account are not here quoted. But at most they would be represented by units, and therefore do not affect the general result. Lastly, the remarkable fact will be noted, that there is no single representative of the mammalian cla.s.s in any of these islands.
If we turn next to consider the case of plants, we obtain the following result:--
+-----------+-----------+--------------+
_Peculiar
_Non-peculiar
Species._
Species._
+-----------
-----------
--------------+
Sandwich