INTRODUCTION. xl 
In some instances, as in the Dermaleichide, the passive migration on birds’ wings 
is no doubt the usual mode of dispersion; and as the regions of Central America 
form a sort of rendezvous and winter station for many birds of passage which are 
infested by Dermaleichide, contact and interchange of Acarid forms of this particular 
group are continually taking place. Much less extensive, though yet considerable, 
is probably the passive migration and the dispersion of those Acarids which, in some 
stages of their existence, adhere to insects and to bats, and which, therefore, are carried 
by insects or on bats’ wings, such as various Tyroglyphide, Gamaside, Trombidide, 
and Hydrachnide; the Hydracnnide attach themselves to Hemipterous water- 
insects which at night abandon their ponds and take wing. In the course of many 
successive generations these Acarids may thus spread over large areas. Amongst the 
non-parasitic species, the active migration of those of open habits and rapid loco- 
motion, such as the Trombidide, Actinedide, &c., may influence the dispersion of 
types and the mutual penetration into different faunas. How far the aerial trans- 
portation by wind and storm, which are such powerful agencies in the migration of 
winged insects and young spiders, may facilitate the passive migration of Acarids is, 
as yet, entirely unknown. 
But, taken as a whole, the various modes of migration, numerous as they are, 
only serve to explain the similarity of types in more or less contiguous land-areas, 
such as North, Central, and South America, and the wide range of single species. 
The almost universal occurrence, however, of certain genera, such as Ixodes, Argas, 
Actineda, Trombidium, Rhyncholophus, Holostaspis, Oribata, Atax, and the world- 
wide distribution of the fundamental types of Acarids, must have another and more 
general cause. ‘This, most probably, is owing to the early dispersion of the primary 
Acarid types from their centres of origin, and in the comparative persistency of 
those types, due to a relatively perfect correspondence between the once acquired 
differentiation of their essential organs and their modes of life. Bearing in mind the 
fact that the local faunas of two so very different and widely separated regions as 
Central Europe and Central America possess a comparatively large number of identical 
generic types and of closely allied species of Acarids, we have perhaps a right to 
generalize and to presume :— 
(1) That this uniformity is, geologically speaking, very ancient, and originated 
in a comparatively early geological period when the relative positions of the continents, 
the islands, and the seas were altogether different from what they are now. 
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