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Show 328 GEOGRAPHICAL DISTRIBUTION, [CHAP. XI. ralists as specifically distinct, by others as varieties ; but some are certainly identical, and many, though closely related to northern forms, must be ranked as distinct species. Now let us see what light can be thrown on the fore-going facts, on the belief, supported as it is by a large body of geological evidence, that the whole world, or a large part of 1t, was, during the Glacial period, simultaneously much colder than at present. The Glacial period, as measured by years, must have been very long ; and when we rmnen1ber 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 tmnperate 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 1nany species as we see at the present day crowded together at the Cape of Good Hope, and in parts of temperate Ausstralia. As we know that many tropical plants and animals can withstand 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 clhnate 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 presented a firm front against intruders, that a certain number of the more vigorous and d01ninant temperate forms might have penetrated the native ra~s, and have reached or even crossed the equator. Them- CHAP. XI.] DURING THE GLACIAL PERIOD . . 329 vas1on would, of course hav b high land, and perhaps' b a e d een .greatly favoured by coner informs me that it i!th JY chm~te; for Dr. Fal- . tropics, which is so destructiv~ tamp, Wlt~l the heat of the temperate climate. On the oth 0 ~phre~nial plants from a and hottest districts will have am ·da~ ' the most humid f:j'opicalnatives. The mounta· 01 e an asylum to the Himalaya, and the lonoo line ~~ranges no~th-west of the have afforded two gre~t line 0 ;~e Co;-d1llera, seem to striking fact, lately communic:t ~ tinvaslon: and it is a that all the :flowering plants ab t £me b;r J?r. liooker, common to Tierra del Fue 0 ' ou orty-s1x I?- nu:r;nber, North America, which m~sth~;o ~~rope still e~1st in march. But I do not doubt th t e ain on the hne of tions entered and crossed even tl s~mel tedperate producat the period when the cold wa Ie o~. an s of the tropics tic forms had mi rated s mos Intense,-when arc-tude from their !tive c~:~~rytwenJy-:five dedgrees of latithe foot of the p renees A ~n c?vere the land at I believe that th!climat~ unde~h~~ penod of extreme cold, of the sea was ab t th . e equator at the level the height of six ~:seve~s:~~s:~~ ft~~t nnw ~elt tt~re at coldest period I suppose that 1 · unng t 1s the lowlands wer~ clothed with a~ge ls~aces of the tropical perate vegetation like that mlng e t;opical and ternluxuriance at the base of th H?W lgrowing 'vith strange scribed by I-Iooker.' · e lma aya, as graphically de- Thus, as I believe a c · d bl few terrestrial animal's anonsi era e ?umber of plants, a grated during the Gl~ciald so:n~ fanne productions, misouthern tern erate peno ro.m the northern and and some eveK cross:~nthe ~~qto tfe lnAtertropical regions, turned the t ua or. s the warmth rethe hi' her s~ emp~rate forms would naturally ascend lands ·gthose .::hP·ti}s,dbmng exterminated on the lowre- mi~ate north~ardl~r not reached the equator, would homes; but the forms chi~fluthward towar?s their former ~he equator, would t;av I rRo;thelrn, Whlch ha~ crossed mto the more tern e s .I urt Ier from thmr homes sphere. Altho hperath latitudes of the opposite hemi-ug we ave reason to believe from geo- |
