THE DISPERSAL OF SPIDERS
III. THE DISPERSAL OF SPIDERS.
In the year that L. Paulus and C. Marcellus were consuls, it rained wool.
For several years L. Berland and I have been in disagreement as to
the relative importance of the different factors which influence
distribution. The friendly nature of this contest has been such that
should either of us in the future be converted to the other's cause there
would remain, I feel certain, nothing but pleasure for both of us at the
realization that our joint efforts had ultimately brought us to the same
Berland believes that the distribution of species today can be
explained primarily in terms of (1) former land connections and (2)
the unwitting transportation of spiders by man in his cargo and ships.
My views, on the other hand, are (1) that many of the assumed land
bridges vanished, if they ever existed, too long ago to serve as an
explanation of the present distribution of species, (2) that man has
undoubtedly provided the agency by which many species have been
introduced from one country to another, and (3) that a more important
factor is the constant interchange of species between neighbouring
lands by air.
A. LAND BRIDGES.
Out of the hundreds of species which have been found embedded in
Oligocene amber not one, it seems, has survived till the present day,
so it follows that if importance is to be placed on land bridges these
bridges must have been in existence at least until Miocene times. In
point of fact the changes in climate which have occurred since
Miocene times over a large part of the globe make it