No. 1.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 73 



these same types, also the Ailantus, Giugko. and a goodly number of coniferous genera. 

 I cannot point to any types tending to make np the deficiency; that is, to any not 

 either in East North America or in Northeast 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 type 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 recognizable distinctions between fossil remains and living 

 aualogues. Prol)ably the European Miocene forest was about as rich and various as 

 is ours of the present day, and very like it. The Glacial i)oriod came and paased, and 

 these types have not survived there, nor returned. Hence the comparative poverty 

 «f the existing European sylva, or, at least, the probable explanation of the absence 

 of those kinds of trees which make the characteristic difference. 



Why did these trees perish out of Europe, bnt survive in America and Asia ? Before 

 ■we inquire how Europe lost them, it may be well to ask how it got them. How came 

 these American trees to be in Europe ? And among the rest, how came Europe to 

 have Sequoias, now represented only by our two Big-trees of California ? It actually 

 possessed two species and more — one so closely answering to the Redwood of the coast 

 ranges, and another so very like the Sequoia gigantea of the Sierra Nevada, that, if 

 such fossil twigs with leaves and cones had been exhumed in California instead of 

 Europe, it would confidently be affirmed that we had resurrected the veritable ances- 

 tors of our two giant trees. Indeed, so it may probably be. " Ccehim non animam 

 mutant," &c., may be apj)licable even to such wide wanderings and such vast inter- 

 vals of time. If the specific essence has not changed, and even if it has suffered some 

 change, genealogical connection is to be inferred in all such cases. 



That is, in these days it is taken for granted that individuals of the same sjiecies, 

 or with a certain likeness throughout, had a single birthplace and are descended from 

 the same stock, no matter how widely separated they may have been either in space 

 or time, or both. The contrary suiiposition may be made, and was seriously enter- 

 tained by some not very long ago. It is even supposable that i)lants and animals 

 originated where they now are, or where their remains were found. But this is not 

 science — in other words, it is not conformable to what we now know, and is an asser- 

 tion that scientific explanation is not to be sought. 



Furthermore, when species of the same genus are not found almost everywhere, 

 they are usually grouped in one region, as are the Hickories in the Atlantic States, 

 the Asters and Goldenrods in North America and prevailingly on the Atlantic side, the 

 heaths in Western Europe and Africa. From this we are led to the inference that all 

 species closely related to each other have had a common birthplace and origin. So 

 that, when we find individuals of a species or of a group widely out of the range of 

 their fellows we wonder how they got there. When we find the same species all round 

 the hemisphere, we ask how this dispersion came to pass. 



Now, a very considerable number of species of herbs and shrubs and a few trees 

 of the temperate zone are found all round the northern hemisphere ; many others are 

 found part way round — some in Europe and Eastern Asia, some in Europe and our 

 Atlantic States, many, as I have said, in the Atlantic States and Eastern Asia — fewer 

 (which is curious) common to Pacific States and Eastern Asia, nearer though these 

 counti'ies be. 



We may set it dt)wn as useless to try to account for this distribution by causes now in 

 operation and opportunities now afiorded, /. e., for distribution across oceans by winds 

 and currents and birds. These means play their part in dispersion from place to 

 place, by step after step, but not from continent to continent, except for few things 

 and in a subordinate way. 



Fortunately we are not obliged to have recourse to overstrained suppositions of 

 what might possibly have occurred now and then, in the lapse of time, by the chance 



