480 Proceedings of Royal Society of Edinburgh. [sess. 
northern continents. The presumption seems that this connection 
was by way of America, and the distribution of some allied 
springtails supports this presumption.* The common European 
Isotoma palustris , Muller, occurs both in North and South 
America; and Schaffer (1897) has described an Isotoma—/. 
obtusicauda — from Valparaiso, closely allied to two peculiar 
northern species, I. crassicauda, Tullberg, and I. litter alis, Della 
Torre. These last-mentioned insects come nearer than any other 
species of Isotoma to I. Brucei and I. Beselsii , agreeing with them 
in the evident position of the spring on the fourth abdominal seg- 
ment, but differing in the absence of prominent teeth on the 
mucrones. We find, therefore, that these groups of springtails, 
considered until a few years ago characteristically Arctic and sub- 
Arctic, are represented in the Andean sub-region of South America, 
in Tierra del Euego, and in the distant South Orkney Islands. 
Must I. Brucei , with its northern affinities, be regarded as an 
older or a newer member of the South Orcadian fauna than the 
distinctively Antarctic species that share its present home 1 
Northern species, at or beyond the southern limits of the present 
American continent, must be either comparatively recent immi- 
grants — Pliocene or later — or else carry us back to early Mesozoic 
times ; for the existence of some sea-channel across America, 
checking migration from north to south, during the Cretaceous 
and Early Tertiary periods, is generally admitted. Von Jhering, 
for example, lays stress (1891) on the faunistic distinction between 
southern and northern South America, and suggests the existence in 
Secondary and Early Tertiary times of two continents — an “ Arclii- 
plata ” connected with Antarctica, and an “ Archicguyana ” con- 
nected by an Atlantis with West Africa. Now it seems unlikely 
that I. Brucei can be a late Tertiary immigrant into the Antarctic 
regions. The necessary connection of the South Orkneys with 
Patagonia can hardly have lasted late enough. And the group to 
which the species belongs is a primitive group even of this com- 
paratively primitive genus and order. In these insects, as 
mentioned above, the spring evidently belongs to the fourth 
abdominal segment, whereas in most species of the genus and 
* Which receives unexpected confirmation from Wahlgren’s discovery (1906) 
of I. Beselsii in Tierra del Fuego. 
