XIV INTRODUCTION. 
whether it is of a more recent origin, whether its various families, some of which are 
not very closely allied to each other, took their origin from one or from several types, 
are questions we shall probably never be able to answer. Arachnoid Arthropods 
appear early in the strata of the primary periods, and it is quite possible that Acaroid 
types were among them, though the delicacy and minuteness of their structure made 
their preservation highly improbable. It is even possible that a closer examination 
of those sedimentary layers which are fine enough for the preservation of more delicate 
organisms, such as some of the tertiary strata (those of Oeningen for instance), may 
lead to the discovery of the larger and more chitinized forms, such as the Ixodide, 
Gamaside, and Oribatide. At present only one Acarid species is known from the 
Tertiary deposits of Oeningen. A larger number of Acarid types have been described 
from the Balticamber. The brown coal of Rott and the Green River beds of Wyoming 
have furnished a few isolated forms. 
But though, as yet, any direct proofs of the geological antiquity of the Acarid type 
beyond the Oligocene are wanting, the above-quoted fact of a most extensive geogra- 
phical distribution of the principal genera, and the general uniformity and similarity 
of the European and extra-European local faunas, as far as they are known at present, 
are highly in favour of a pretertiary origin of the Acarid types. 
If we compare the Acarid fauna of those parts of Europe where it has been some- 
what carefully studied with that of Central America, we are compelled to say that 
Central America is comparatively poor as regards the number of species, far more 
so than we should be inclined to anticipate when we take into consideration its great 
variety of soil and climate and its general richness in vegetable and animal productions. 
I willingly admit that the districts to which my personal researches were confined 
are not very extensive, and perhaps other parts of Guatemala, such as the high valleys 
of Alta Vera Paz, or the forests of the alpine mountains of the “ Altos,” or even the 
richer slopes of the Atlantic coast, may have a more varied Acarid fauna; but as the 
various collectors in other parts of Central America did not, in so conspicuous a 
group as the Oribatide, meet with any other species than those which I obtained 
in Guatemala, it seems to me probable that even those parts, when searched more 
carefully, will not prove very much richer or more varied in Acarid forms, though 
undoubtedly they may yield some new species which escaped my notice. I am therefore 
inclined to believe that there really exists a comparative scarcity of Acarid species, at 
least in Western Guatemala, and that the principal cause of this is to be found in 
