1903.] GEOGRAPHICAL DISTRIBUTION OF SPIDERS. 341 
nothing is known of the forms that inhabited the world during 
the enormous lapse of time represented by the Mesozoic strata, 
and nothing except inferentially of the types that occupied the 
southern countries of the world during Tertiary times. 
Spiders of the type that lived in the Carboniferous period 
succeeded in holding their own in Europe until the Oligocene, and 
are represented at the present time by the genus Liphistius, which 
is restricted to the Indo-Malayan area of the Oriental Region. 
Apart from the genus Liphistius, all existing Spiders, including 
the Mygalomor phe, belong to the group Opisthothele. There is no 
evidence that this group existed in the Carboniferous period ; but 
since most of the Oligocene and Miocene fossils belong to existing 
families, or sometimes indeed to existing genera, It is permissible 
to suppose that the Opisthothele originated some time during the 
Mesozoie epoch, and may, in fact, be. coeval with the mammalia. 
Whether any of these hypothetical Mesozoic forms survive to the 
present day, it is quite impossible to say. All that paleontology 
allows us to infer is that during the Tertiary period there was 
a rich and varied spider -population spread over the Northern 
hemisphere, contaiing forms that have undergone but little 
metamorphosis since that date. The existence of ‘Mygalomor phe 
at that time is attested by the discovery of one form referred to 
Mygale in the gypsum-beds at Aix, and of another, Hoatypus, 
in the Eocene strata at Garnet Bay in the Isle of Wight. But 
since it is impossible to classify these forms with an approach 
to certainty in any of the existing families, their only value 
from the geographical standpoint is the evidence they supply 
that the Mygalomorphe had come into being in Tertiary times, and 
were living in the Northern hemisphere. 
The imperfections in our knowledge above alluded to permit 
only a provisional acceptance of the theories put forward in the 
following pages to explain the distributional phenomena of the 
Mygalomorphe. But all the available evidence, little enough 
though it be, points to the conclusion that the Mygalomorphe 
and the rest of the Opisthothele appeared first in the Northern 
hemisphere, and spread thence over the southern countries of 
the world, 
(6) Means of Dispersal of Spiders, and the importance of the 
Mygalomorphe from the Geographical standpoint. 
It cannot be claimed that Spiders as a whole are a favourable 
group to study from a geographical point of view; for, although 
exclusiv ely terrestrial when adult, and, like other flightless animals, 
dependent upon continuity of land-surfaces for migration, a great 
many species are known to have the power, and the instinct to 
put it in foree, of dispersing themselves over wide areas by 
practising when young the habit of flight, using silk-threads as 
aerial floats upon which they may be carried long distances before 
the wind. This phenomenon is well known, and has given rise to 
