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82-7, gives the case of apterous insects, but I remember I worked out
some additional details.
I think he gives in these same works the proportion of European insects.
LETTER 366. TO J.D. HOOKER.
(366/1. Sir Joseph had asked (July 31st, 1866): "Is there an evidence
that the south of England and of Ireland were not submerged during the
Glacial epoch, when the W. and N. of England were islands in a glacial
sea? And supposing they were above water, could the present Atlantic
and N.W. of France floras we now find there have been there during the
Glacial epoch?--Yet this is what Forbes demands, page 346. At page 347
he sees this objection, and wriggles out of his difficulty by putting
the date of the Channel 'towards the close of the Glacial epoch.' What
does Austen make the date of the Channel?--ante or post Glacial?" The
changes in level and other questions are dealt with in a paper by R.A.C.
Austen (afterwards Godwin-Austen), "On the Superficial Accumulations
of the Coasts of the English Channel and the Changes they indicate."
"Quart. Journ. Geol. Soc." VII., 1851, page 118. Obit. notice by Prof.
Bonney in the "Proc. Geol. Soc." XLI., page 37, 1885.)
Down, August 3rd [1866].
I will take your letter seriatim. There is good evidence that S.E.
England was dry land during the Glacial period. I forget what Austen
says, but Mammals prove, I think, that England has been united to the
Continent since the Glacial period. I don't see your difficulty about
what I say on the breaking of an isthmus: if Panama was broken through
would not the fauna of the Pacific flow into the W. Indies, or vice
versa, and destroy a multitude of creatures? Of course I'm no judge, but
I thought De Candolle had made out his case about small areas of trees.
You will find at page 112, 3rd edition "Origin," a too concise allusion
to the Madeira flora being a remnant of the Tertiary European flora.
I shall feel deeply interested by reading your botanical difficulties
against occasional immigration. The facts you give about certain plants,
such as the heaths, are certainly very curious. (366/2. In Hooker's
lecture he gives St. Dabeoc's Heath and Calluna vulgaris as the most
striking of the few boreal plants in the Azores. Darwin seems to have
been impressed by the boreal character of the Azores, thus taking the
opposite view to that of Sir Joseph. See Letter 370, note.) I thought
the Azores flora was more boreal, but what can you mean by saying that
the Azores are nearer to Britain and Newfoundland than to Madeira?--on
the globe they are nearly twice as far off. (366/3. See Letter 368.)
With respect to sea currents, I formerly made enquiries at Madeira,
but cannot now give you the results; but I remember that the facts were
different from what is generally stated: I think that a ship wrecked on
the Canary Islands was thrown up on the coast of Madeira.
You speak as if only land-shells differed in Madeira and Porto Santo:
does my memory deceive me that there is a host of representative
insects?
When you exorcise at Nottingham occasional means of transport, be
honest, and admit how little is known on the subject. Remember how
recently you and others thought that salt water would soon kill seeds.
Reflect that there is not a coral islet in the ocean which is not pretty
well clothed with plants, and the fewness of the species can hardly
with justice be attributed to the arrival of few seeds, for coral islets
close to other land support only the same limited vegetation. Remember
that no one knew that seeds would remain for many hours in the crops of
birds and retain their vitality; that fish eat seeds, and that when the
fish are devoured by birds the seeds can germinate, etc. Remember that
every year many birds are blown to Madeira and to the Bermudas. Remember
that dust is blown 1,000 miles over the Atlantic. Now, bearing all this
in mind, would it not be a prodigy if an unstocked island did not in the
course of ages receive colonists from coasts whence the currents flow,
trees are drifted and birds are driven by gales. The objections to
islands being thus stocked are, as far as I understand, that certain
species and genera have been more freely introduced, and others less
freely than might have been expected. But then the sea kills some sorts
of seeds, others are killed by the digestion of birds, and some would
be more liable than others to adhere to birds' feet. But we know so very
little on these points that it seems to me that we cannot at all tell
what forms would probably be introduced and what would not. I do not for
a moment pretend that these means of introduction can be proved to
have acted; but they seem to me sufficient, with no valid or heavy
objections, whilst there are, as it seems to me, the heaviest objections
on geological and on geographical distribution grounds (pages 387, 388,
"Origin" (366/4. Edition III., or Edition VI., page 323.) to Forbes'
enormous continental extensions. But I fear that I shall and have bored
you.
LETTER 367. J.D. HOOKER TO CHARLES DARWIN.
(367/1. In a letter of July 31st, Sir J.D. Hooker wrote, "You must not
suppose me to be a champion of continental connection, because I am not
agreeable to trans-oceanic migration...either hypothesis appears to
me well to cover the facts of oceanic floras, but there are grave
objections to both, botanical to yours, geological to Forbes'.")
The following interesting letters give some of Sir Joseph's
difficulties.)
Kew, August 4th, 1866.
You mention ("Journal") no land-birds, except introduced, upon St.
Helena. Beatson (Introduction xvii) mentions one (367/2. Aegialitis
sanctae-helenae, a small plover "very closely allied to a species found
in South Africa, but presenting certain differences which entitle it to
the rank of a peculiar species" (Wallace, "Island Life," page 294).
In the earlier editions of the "Origin" (e.g. Edition III., page 422)
Darwin wrote that "Madeira does not possess one peculiar bird."
In Edition IV., 1866, page 465, the mistake was put right.) "in
considerable numbers," resembles sand-lark--is called "wire bird," has
long greenish legs like wires, runs fast, eyes large, bill moderately
long, is rather shy, does not possess much powers of flight. What was
it? I have written to ask Sclater, also about birds of Madeira and
Azores. It is a very curious thing that the Azores do not contain the
(non-European) American genus Clethra, that is found in Madeira and
Canaries, and that the Azores contain no trace of American element
(beyond what is common to Madeira), except a species of Sanicula, a
genus with hooked bristles to the small seed-vessels. The European
Sanicula roams from Norway to Madeira, Canaries, Cape Verde, Cameroons,
Cape of Good Hope, and from Britain to Japan, and also is, I think, in
N. America; but does not occur in the Azores, where it is replaced by
one that is of a decidedly American type.
This tells heavily against the doctrine that joins Atlantis to America |