Fek 6, 1879] 



NATURE 



329 





not only most coniferous trees, but a notably larger number of 

 trees altogether than any other part of the northern temperate 

 zone? Why should its only and near rival be in the antipodes, 

 namely, here in Atlantic North America ? In other words, why 

 should the Pacific and the European forests be so poor in com- 

 parison, and why the Pacific poorest of all in deciduous, yet rich 

 in coniferous trees ? " 



Prof. Gray ventures to conclude that this richness is normal, 

 and that what we really have to explain is the absence of so 

 many forms from Europe on the one hand, from Oregon and 

 California on the other. He shows that most of the forms, even 

 of shrubs and herbs, which are peculiar to the Atlantic forest, 

 have their close counterparts in Japan and North China. Prof. 

 Gray noticed the feature long ago, and evidences of the remark- 

 able relationship have multipli^ year after year. 



"The result, as to the trees, is seen in these four diagrams. 

 As to number of species generally, it cannot be said that 

 Europe and Pacific North America are at all in arrears. But as 

 to trees, either the contrasted regions have been exceptionally 

 favoured, or these have been hardly dealt with. There is, as I 

 have intimated, some reason to adopt the latter alternative. 



"We may take it for granted that the indigenoxxs plants of 

 any country, particularly the trees, have been selected by climate. 

 Whatever other influences or circumstances have been brought 

 to bear upon them, or the trees have brought to bear on each 

 other, no tree could hold its place as a member of any forest 

 or flora which is not adapted to endure even the extremes of the 

 climate of the region or station. But the character of the 

 climate will not explain the remarkable paucity of the trees 

 which compose the indigenous European forest. That is proved 

 by experiment, sufficiently prolonged in certain cases to justify 

 the inference. Probably there is no tree of the northern tempe- 

 rate zone which will not flourish in some part of Europe. Great 

 Britain alone can grow double or treble the number of trees that 

 the Atlantic States can. In all the latter we can grow hardly 

 one tree of the Pacific coast. England supports all of them, 

 and all our Atlantic trees also, and likewise the Japanese and 

 North Siberian species, which do thrive here remarkably in 

 some part of the Atlantic coast, especially the cooler-temperate 

 ones. The poverty of the European sylva is attributable to the 

 absence of our Atlantic American types, to its having no mag- 

 nolia, liriodendron, asimina, negundo, no sesculus, none of that 

 rich assemblage of leguminous trees represented by locusts, 

 honey -locxists, gymnocladus, and cladrastis (even its cercis, which 

 is hardly European, is like tie Calif omian one, mainly a shrub) ; 

 no nyssa, nor liquidambar ; no ericacese rising to a tree ; no 

 bumdia, catalpa, sassafras, osage orange, hickory, or walnut ; 

 and as to conifers, no hemlock spruce, arbor-vitas, taxodium, 

 nor torreya. As compared with north-eastern Asia, Europe 

 wants most of these same types, also the ailantus, gingko, and a 

 goodly number of coniferous genera. I cannot point to any 

 types tending to make up the deficiency, that is, to any not 

 either in east -north America or in north-east Asia, or in both. 

 Cedrus, the [true cedar, which comes near to it, is only north 

 African and Asian, I need not say that Europe has no sequoia, 

 and shares no special tj'pe with California. 



" Now the capital fact is, that many and perhaps almost all of 

 these genera of trees were well represented in Europe throughout 

 the later tertiary times. It had not only the same generic types, 

 but in some cases even the same species, or what must pass as 

 such, in the lack of recognisable distinctions between fossil 

 remains and living analogues. Probably the European miocene 

 forest was about as rich and various as is ours of the present day, 

 and very like it. The glacial period came and passed, and these 

 'types Imve not sur\-ived there, nor returned. Hence the com- 

 parative poverty of the existing Emropean sylva, or at least, the 

 probable explanation of tJie absence of those kinds of trees which 

 make the characteristic difference." 



Before answering the question as to why these trees perished 

 out of Europe but survive in America and Asia Prof. Gray 

 inquires how these American trees came to be in Europe. From 

 certain consideratioiis he is led to the inference that all species 

 closely related to each other have had a common birthplace and 

 jorigin. So that when we find individuals of a species or of a 

 group widely out of range of their fellows, we wonder how they 

 got liere. When we find the same species all round the hemi- 

 sphere — and a verj- considerable number of species of herbs and 

 shrubs, and a few trees are so f oimd — we ask how this dispersion 

 came to pass. Prof. Gray goes on to say : — 



"I take it that the true explanation of the whole problem 



comes from a just general view, and not through piecemeal 

 suppositions of chances. And I am clear that it is to be found 

 by looking to the north, to the state of things at the arctic zone 

 first, as it now is, and then as it has been. North of our forest- 

 regions comes the zone imwooded from cold, the zone of arctic 

 vegetation. In this, as a rule, the species are the same round 

 the world ; as exceptions, some are restricted to a part of the 

 circle. The polar projection of the earth down to the northern 

 tropic, as here exhibited, shows to the eye — as our maps do not 

 — ^how all the lands come together into one region, and how 

 natural it may be for the same species, under homogeneous con- 

 ditions, to spread over it. When we know, moreover, that sea 

 and land have varied greatly since these species existed, we may 

 well believe that any ocean-gaps, now in the way of equable 

 distribution, may have been bridged over. There is now only 

 one considerable gap. 



" What would happen if a cold period were to come on from 

 the north, and were very slowly to carry the present arctic 

 climate, or something like it, down far into the temperate zone ? 

 Why, just what has happened in the glacial period, when the 

 refrigeration somehow pushed all these plants before it down 

 to southern Europe, to middle Asia, to the middle and southern 

 part of the United States ; and at length receding, left some 

 parts of them stranded on the Pyrenees, the Alps, the Apennines, 

 the Caucasiis, on our W^hite and Rocky Mountains, or wherever 

 they could escape the increasing warmth as well Ijy ascending 

 mountains as by receding northward at lower levels. Those that 

 kept together at a low level, and made good their retreat, form 

 the main body of present arctic vegetation. Those that took to 

 the mountains had their line of retreat cut off", and hold their 

 positions on the mountain-tops imder cover of the frigid climate 

 due to elevation. The conditions of these on different conti- 

 nents or different mountains are similar, but not wholly alike. 

 Some species proved better adapted to one, some to another, 

 part of the world ; where less adapted, or less adaptable, they 

 have perished; where better adapted, they continue — with or 

 without some change ; — and hence the diversification of alpine 

 plants, as well as the general likeness through all the northern 

 hemisphere. 



"All this exactly applies to the temperate zone vegetation, 

 and to the trees that we are concerned with. The clue was 

 seized when the fossil botany of the high arctic r^ons came 

 to light ; when it was demonstrated that in the times next pre- 

 ceding the glacial period — in the latest tertiary — from Spitzbergen 

 and Iceland, to Greenland and Kamtschatka, a climate like 

 that we now enjoy prevailed, and forests like those of New 

 England, and Virginia, and of California, clothed the land. We 

 infer the climate from the trees ; and the trees give sure indica- 

 tions of the climate. 



"I had divined and published the explanation long before I 

 knew of the fossil plants. These, since made known, render the 

 inference sure, and give us a clear idea of just what the climate 

 was. At the time we speak of, Greenland, Spitzbergen, and 

 our arctic sea-shore had the climate of Pennsylvania and 

 Virginia now. It would take too much time to enumerate the 

 sorts of trees that have been identified by their leaves and fruits 

 in the arctic later tertiary deposits. 



" . . . . Long genealogies always deal more or less in con- 

 jecture ; but we appear to be within the limits of scientific 

 inference when we announce that our existing temperate trees 

 came from the north, and within the bormds of high probability 

 when we claim not a few of them as the originals of present 

 species. Remains of the same plants have been found fossil in 

 our temperate region, as well as in Europe. 



" Here, then, we have reached a fair answer to the question 

 how the same or similar species of our trees came to be so dis- 

 persed over such widely separated continents." 



Prof. Gray then shows what would naturally follow from a 

 gradual pushing of the Arctic vegetation southwards, and that 

 the modifications resulting from differences of climate in the 

 divergent continents, and on their different sides, might well 

 account for the present diversification. The siftings and resift- 

 ings which have since taken place from changes of climate, sub- 

 mergence, and re-emergence, and other causes, have left their 

 impress on the actual v^etation, especially on the trees. They 

 furnish probable reason for the loss of American types sustamed 

 by Europe. 



" I conceive that three things have conspired to this loss. 

 First, Europe, hardly extending south of latitude 40°, is all 

 withhi the limits generally assigned to severe glacial action. 



