7 ee INTRODUCTION. 
introduced with the cargo carried by ships in recent times. Where the forms are 
identical in two widely separated regions, this will, no doubt, account for their 
presence in both; but the time required for the specialization of species precludes 
the idea that this agency is in any way responsible for forms representing merely 
closely allied species generically identical. 
In order to find a satisfactory explanation of this startling resemblance between the 
fauna of Central America and that of the regions mentioned above, we shall have to 
take into consideration the geological history of the Araneidea, wherever possible, 
and also the past geographical history of the various continents as we now know 
them. 
The Araneidea, of which the Mygalomorphe are the modern exponents, date back 
at least as far as the Carboniferous period, when they were represented by forms of 
which the two species of Liphistius are the sole survivors. ‘These differ considerably 
from the rest of the Mygalomorphe, since their spinners lie in the middle of the 
underside of the abdomen, instead of at the apex. The earliest known Arachnomorphid 
spiders, however, in any way resembling those now living belong to the Oligocene 
times. At this period there existed many species, very similar to those occurring at 
the present time, whose remains have been found embedded in amber washed up on 
the shores of the Baltic Sea. Beyond these comparatively few examples, we know 
little of the extinct races of the Araneidea. 
That the spiders comprised in the Central-American fauna could not during recent 
times have passed to and fro between that region and the great eastern continents will 
be obvious from a glance at the distribution of land and water at the present day. 
‘There are now in existence no land-connections of which an Arachnidal fauna could 
take advantage in order to pass from Africa, Australia, or the Palearctic Region into 
North or South America. We can only conclude that at some period or other | 
these now widely separated regions must have been linked together by land which 
has long since disappeared. 
As illustrations of these resemblances in the fauna we may note the distribution of 
a few well-known forms. ‘Taking, first, the Mygalomorphe, which includes the most 
primitive Araneidea, we find the family Dipluride represented in Central America, 
the Antilles, Western Ethiopia, Madagascar, the Mediterranean Region, India, the 
