atus diaboli has a useful
function in science. But in attacking Darwin his brief is generally
found to be founded on a slender basis of facts. Thus Winge and Knud
Andersen have examined many thousands of migratory birds and found "that
their crops and stomachs were always empty. They never observed any
seeds adhering to the feathers, beaks or feet of the birds." (R.F.
Scharff, "European Animals", page 64, London, 1907.) The most
considerable investigation of the problem of Plant Dispersal since
Darwin is that of Guppy. He gives a striking illustration of how easily
an observer may be led into error by relying on negative evidence.

"When Ekstam published, in 1895, the results of his observations on the
plants of Nova Zembla, he observed that he possessed no data to show
whether swimming and wading birds fed on berries; and he attached all
importance to dispersal by winds. On subsequently visiting Spitzbergen
he must have been at first inclined, therefore, to the opinion
of Nathorst, who, having found only a solitary species of bird (a
snow-sparrow) in that region, naturally concluded that birds had been
of no importance as agents in the plant-stocking. However, Ekstam's
opportunities were greater, and he tells us that in the craws of six
specimens of Lagopus hyperboreus shot in Spitzbergen in August he found
represented almost 25 per cent. of the usual phanerogamic flora of that
region in the form of fruits, seeds, bulbils, flower-buds, leaf-buds,
etc... "

"The result of Ekstam's observations in Spitzbergen was to lead him to
attach a very considerable importance in plant dispersal to the agency
of birds; and when in explanation of the Scandinavian elements in the
Spitzbergen flora he had to choose between a former land connection and
the agency of birds, he preferred the bird." (Guppy, op. cit. II. pages
511, 512.)

Darwin objected to "continental extensions" on geological grounds,
but he also objected to Lyell that they do not "account for all the
phenomena of distribution on islands" ("Life and Letters", II. page
77.), such for example as the absence of Acacias and Banksias in New
Zealand. He agreed with De Candolle that "it is poor work putting
together the merely POSSIBLE means of distribution." But he also
agreed with him that they were the only practicable door of escape from
multiple origins. If they would not work then "every one who believes
in single centres will have to admit continental extensions" (Ibi