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Show 376 GEOGRAPHICAL DISTRIBUTION, CHAP. XI. identically the same; but they are much ofte~er specifically distinct, though related to each other In a most remarkable manner. This brief abstract applies to plants alone :. s~me strictly analogous facts could be given. on the dist~Ibu-ti. o n of t errest n. · al animals · In marine productiOns, si·m i·1 ar case s occur ·' as an ex. ample, I may qhu ote" ·a remark by the highest authonty, Prof. Dana, t at It I·S cer t a1· n1 y a wonderful fact. th.a t New Zealand should have a closer resemblance m Its crustacea to Great Britain, its antipode, than to any other part of the world." Sir J. Richardson, also, speaks of the I:e-pearance on the shores of New Zealand, Tasmania, :., of northern forms of fish. Dr. Hooker informs me that twenty-five species of Algoo are common to New Zealand and to Europe, but have not been found in the intermediate tropical seas. . It should be observed that the northern spemes an~ forms found in the southern parts of the so~thern he~msphere, and on the mountain-ranges of the IntertropiCal regions are not arctic, but belong to the northern temperate 'zones. As Mr. H. C. Watson has recently ~emarked, "In receding from polar towards equatonal latitudes, the Alpine or mountain floras re.a~y become less and less arctic." Many of the forms hving on t~e mountains of the warmer regions of the earth and. m the southern hemisphere are of doubtful valu~, bemg ranked by some naturalists as specifically dis~Inct,. by others as varieties; but some are certainly Identical, and many, though closely rel~ted to northern forms, must be ranked as distinct specws. Now let us see what light can be thro~.on the fore-going facts, on the belief, supported as It IS by a la:g: body of geological evidence, that the. whole. wor~d, o - large part of it, was during the Glacial perwd simulta CHAP. XI. DURING THE GLACIAL PERIOD. 377 neously much colder than at present. The Glacial period, as measured by years, must have been very long; and when we remember over ·what vast spaces some naturalised plants and animals have spread within a few centuries, this period will have been ample for any amount of migration. As the cold came slowly on, all the tropical plants and other productions will have retreated from both sides towards the equator, followed in the rear by the temperate productions, and these by the arctic; but with the latter we are not now concerned. The tropical plants probably suffered much extinction; how much no one can say; perhaps formerly the tropics supported as many species as we see at the present day crowded together at the Cape of Good Hope, and .in parts of temperate Australia. As we know that many tropical plants and animals can vdthstand a considerable amount of cold, many might have escaped extermination during a moderate fall of temperature, more especially by escaping into the warmest spots. But the great fact to bear in mind is, that all tropical productions will have suffered to a certain extent. On the other hand, the temperate productions, after migrating nearer to the equator, though they will have been placed under somewhat new conditions, will have suffered less. And it is certain that many temperate plants, if protected from the inroads of competitors, can withstand a much warmer climate than their own. Hence, it seems to me possible, bearing in mind that the tropical productions were in a suffering state and could not have pre sen ted a firm front against intruders, that a certain number of the more vigorous and dominant temperate forms might have penetrated the native ranks and have reached or even crossed the equator. The invasion would, of course, have been greatly favoured by high land, and perhaps |