10 INFLUENCE OF CLIMATIC Atfb GEOLOGICAL CHANGES 



wooded in the Tertiary Period. Forbes cited the Gulf-weed, 

 Siir&issum laccijerum, in support of his theory of a lost Atlantis, 

 which is met with in mid-ocean. Sir Joseph Hooker decides it to 

 be an abnormal condition of S. vulgare, a coast-line plant grow- 

 ing on vertical rocks, and within a very limited range, and 

 accounts for its abnormal variety by the submergence of an 

 ancient line of coast, that it is not propagated by fructification, 

 but by breakage, the young shoots pushing out from the detached 

 fronds becoming matured. He proposed to call it Sargassum 

 Imccifcnim. It seems, therefore, not an improbable conjecture 

 that the Gulf-weed indicates that land had been submerged, 

 and that its presence must always be taken into account 

 in any speculations on the past changes of the distribution 

 of land and sea. In none of the strata of a date anterior 

 to the Glacial Age have the remains of a boreal plant or 

 animal been met with. Life adapted to the rigours of cold 

 appears to have been developed for the contingency. Mice 

 and rats abound in the tropics, but are nowhere found in 

 the Arctic Circle, where Voles, which are absent in the tropics, 

 abound. They both meet in the warm temperate regions. Mice 

 are found in Tertiary beds, but not the Vole, until the subsequent 

 deposition of the diluvium at the commencement of the Glacial 

 Period. Nothing which remained in the Northern Hemisphere 

 could then escape alteration, except whatever moved southward. 

 Many geologists view the polished and scratched rock-surfaces 

 beneath the boulder-clay as evidence of a former extension of 

 land-ice, and that the counties of Kent, Surrey, Sussex, Hants, 

 Dorset, Somerset, Devon, and Cornwall, and a part of Wiltshire 

 were comparatively free of it. In temperate latitudes ice-sheets 

 and glaciers, unlike those in Arctic regions, terminate inland, as 

 is the case in Switzerland, the Himalayas, and in America. 

 During the alterations of warm interglacial periods (how many 

 they were we do not know) a return of animals and plants 

 northward took place. 



That there was more than one migration is shown by the 

 Boulder Clay, an unstratified mass charged with angular, 





