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Show 1 9 0 3 . ] GEOGRAPHICAL DISTRIBUTION OF SPIDERS. 341 nothing is known of the forms that inhabited the world during the enormous lapse of time represented by the Mesozoic strata, and nothing except inferentially of the types that occupied the southern countries of the world during Tertiary times. Spiders of the type that lived in the Carboniferous period succeeded in holding their own in Europe until the Oligocene, and are represented at the present time by the genus Liphistius, which is restricted to the Indo-Malayan area of the Oriental Region. Apart from the genus Liphistius, all existing Spiders, including the Mygalomorphse, belong to the group Opisthothela?. There is no evidence that this group existed in the Carboniferous period ; but since most of the Oligocene and Miocene fossils belong to existing families, or sometimes indeed to existing genera, it is permissible to suppose that the Opisthothelag originated some time during the Mesozoic epoch, and may, in fact, be coeval with the mammalia. Whether any of these hypothetical Mesozoic forms survive to the present day, it is quite impossible to say. All that palaeontology allows us to infer is that during the Tertiary period there was a rich and varied spider-population spread over the Northern hemisphere, containing forms that have undergone but little metamorphosis since that date. The existence of Mygalomorphse at that time is attested by the discovery of one form referred to Mygale in the gypsum-beds at Aix, and of another, Eoatypus, in the Eocene strata at Garnet Bay in the Isle of Wight. But since it is impossible to classify these forms with an approach to certainty in any of the existing families, their only value from the geographical standpoint is the evidence they supply that the Mygalomorphse had come into being in Tertiary times, and were living in the Northern hemisphere. The imperfections in our knowledge above alluded to permit only a provisional acceptance of the theories put forward in the following pages to explain the distributional phenomena of the Mygalomorphpe. But all the available evidence, little enough though it be, points to the conclusion that the Mygalomorphse and the rest of the Opisthothelae appeared first in the Northern hemisphere, and spread thence over the southern countries of the world. (b) Means of Dispersal of Spiders, and the importance of the Mygalomorphce from the Geographical standpoint. It cannot be claimed that Spiders as a whole are a favourable group to study from a geographical point of view ; for, although exclusively terrestrial when adult, and, like other flightless animals, dependent upon continuity of land-surfaces for migration, a great many species are known to have the power, and the instinct to put it in force, of dispersing themselves over wide areas by practising when young the habit of flight, using silk-threads as aerial floats upon which they may be carried long distances before the wind. This phenomenon is well known, and has given rise to |