Page:Supplement to harvesting ants and trap-door spiders (IA supplementtoharv00mogg).pdf/119

 existing forms of life will be traced with sufficient accuracy to enable us to follow on the map the lines along which affinity travels; and thus point out at once the probable relationship between two given forms, and also the route by which they reached their present stations. Records of local varieties, and the careful discrimination between forms which have small but permanent points of difference, thus acquire an importance which they would not otherwise possess.

The geographical distribution of trap-door spiders is of peculiar interest on account of the sedentary habits maintained during life by the females. Most animals are capable of travelling long distances, or of being accidentally transported from place to place in such a way that colonies are frequently established far away from the parent settlement, and we are left in the dark as to whence they came and who are their nearest relations. But, in the case of spiders inhabiting true trap-door nests, this is not so; they begin life immediately on leaving the parent nest by making homes for themselves near at hand which they will not desert, and there is no likelihood of their being accidentally carried from place to place unless occasionally by running water. Thus it happens that whenever we find the same trap-door spider at two distant localities, we may feel tolerably sure that the species has travelled from one to the other by gradual extension, and that, either now or in times past, it occupied all the intervening country.

For instance, we find Nemesia Eleanora at Mentone, and again at Cannes, while it has not yet been detected at Nice, Antibes, nor any other intermediate point; but according to this hypothesis, this species