xil INTRODUCTION. 
(2) That the Acarids long ago arrived at that degree of organic development 
(progressive or regressive) which was the fittest for their various modes of life, not 
partaking in the great and rapid changes of generic and specific characters which, in 
the course of the more recent geological epochs, have more or less affected so many of 
the higher organized types of the animal kingdom. 
The presence of Megisthanus in Central America, a well-characterized Gamasid 
genus which has not yet been found in any of the extratropical regions, is a fact of 
peculiar interest. In his original paper Signor T. Thorell * described three species 
from Java and two from New Guinea. One of the latter (1M. testudo, Thor.) has 
also been mentioned by Signor G. Canestrinif as occurring in Queensland. One 
species has since been added by Signor A. Berlese from Paraguay, and most 
probably the “‘Gamase géant” of A. Dugés§, from Brazil, belongs to the same 
genus. I have already had occasion to mention the fact that this remarkable type 
also occurs in tropical Africa ||. It therefore belongs to the tropical regions of not 
less than four different zoo-geographical areas, viz.: the Oriental (Java), the Australian 
(New Guinea and Australia), the Neotropical (Central and South America), and the 
“Ethiopian (Gold Coast)—a fact which it would be very difficult to explain by a 
migratory dispersion of recent origin from one starting-point. It is far more 
reasonable to regard these now so widely dispersed Megisthani as the surviving 
members of a once, that is in former geological periods, coherent group of Gamasids 
which have been separated in consequence of the slow but material changes of the 
earth’s surface, principally by the successive breaking down of large masses of the 
earth’s crust and the filling up of the thus formed gulfs by the seas. 
The genus Megisthanus is by no means the only example of the occurrence of one 
and the same animal type at different regions which at the present time are separated 
by large tracts of sea, and which for long periods have not had any direct land 
communication whatever with one another. 
* T. Thorell, Descrizione di alcuni Aracnidi inferiori dell’ Arcipelago Malese, 1882. 
t+ G. Canestrini, Acari nuovi o poco noti, p. 14 (Atti del R. Istituto Veneto di Scienze, Lettere ed Arti, 
t. ii. ser, vi. 1884). 
t A. Berlese, Acari Austro-Americani, 1888. 
§ A. Dugés, Recherches sur l’ordre des Acariens, 3° Mém., 1837. 
|| See above, p. x, note. 
