Abstract

 The awareness and study of eastern North American - eastern Asian plant disjunctions is traced from the time of Linnaeus to the beginning of the twentieth century. Thomas Nuttall's previously overlooked contribution to this topic is discussed, and an annotated appendix of the taxa that he attributed to eastern Asia and North America is presented. Charles Darwin's influence on the thoughts and writings of Asa Gray is also discussed, based on published and unpublished letters and manuscripts in the Archives of the Gray Herbarium of Harvard University; the role that this correspondence played in the development of Gray's phytogeographical ideas and Darwin's theory of evolution is considered. A brief summary of the writings of Adolf Engler, and other nineteenth century botanists, pertaining to these disjunctions and their bearing on an understanding of vegetational patterns in the northern hemisphere is also given. The significance of the late nineteenth century explorations in China in making known the full extent of disjunctions between eastern Asia and North America has provided the basis for further work beyond the exploratory and theoretical stages of this fascinating pattern of plant disjunction.

The discontinuous distribution of the same or closely related taxa of plants between eastern North America and eastern Asia is but one of several patterns of disjunction that become evident when the flora of the temperate Northern Hemisphere is considered (Raven, 1972; Thorne, 1972; Wood, 1972; and others). Many of these patterns have been termed Tertiary relict disjunctions. Of these, however, the "classic" eastern North American - eastern Asian pattern is undoubtedly the best known and most often cited example of the disjunct occurrence of closely related taxa on two continents separated by thousands of kilometers. Despite the fact that most of the shared taxa in the two regions have been shown to be distinct from one another (the relationships are primarily closely related species of the same genus, or closely related genera of the same family), other factors noted by biogeographers that have strengthened the impression of a biological connection between the two regions are climatic and ecological similarities. The similarities of the forests of Japan, central China, and the southern Appalachians in appearance as well as in ecological associations are in many instances so great that a sense of déjà vu is experienced by botanists by one of the regions visiting the other.

Another important factor that has served to emphasize the floristic relationships between eastern North America and eastern Asia is the fact that this particular disjunction pattern was the first to be recognized by botanists. Moreover, the discovery and significance of this pattern figured in the discussions surrounding Darwin's theory of evolution, and it has been discussed not only by plant geographers but by botanical and scientific historians as well. The purpose of this paper is to trace briefly the historical aspects of the recognition of this distribution pattern and the accumulation of facts pertaining to it, and to summarize the major contributions to the study of this pattern that have been made from the time of Linnaeus to the beginning of the twentieth century. Special mention of the contribution and observations of Thomas Nuttall is in-

[1] We thank M. Byrnes for her help in the preparation of the manuscript, O. T. Solbrig, Director of the Gray Herbarium, for allowing access to the Darwin and Gray material in the Archives of the Gray Herbarium, and L. McWood, Archivist, for her patience and helpfulness. We also thank D. Kohn, B. G. Schubert, and P. F. Stevens for reading the manuscript and for their comments, and P. H. Raven, who suggested that we attempt this contribution. Lastly, we are grateful to B. Bartholomew and the library of the California Academy of Sciences for providing a copy of Miquel's paper of 1868, and to P. H. Raven and the Missouri Botanical Garden for providing copies of other literature that we were lacking.

[2] Harvard University Herbaria, 22 Divinity Avenue, Cambridge, Massachusetts 02138.

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cluded inasmuch as his penetrating insights into the relationships between the eastern Asian and eastern North American floras, as well as his phytogeographical observations, have to our knowledge hitherto passed unnoticed.

Early Recognition of the Floristic Similarities Between Eastern North America and Eastern Asia - The Linnaean Era

 The history of the early recognition of the floristic similarities between eastern Asia and eastern North America has been well outlined by Graham (1966, 1972b) and Li (1952, 1955). While credit for the discovery of the floristic relationships has frequently been given to Asa Gray, it was pointed out by M. L. Fernald (1931) that the first published reference to these similarities appears in a Linnaean dissertation, Plantae Camschatcenses Rariores, that was published in 1750 by Linnaeus's student Jonas P. Halenius. As discussed by Graham (1966), the dissertation, including the listing of nine species then thought to occur in both Kamchatka and North America, was probably written by Linnaeus himself and not his student, who was apparently only required to defend the dissertation in Latin.

In 1751, the year after the initial publication of Halenius's dissertation, Pehr Kalm, another Swedish botanist, who had traveled extensively in North America during the years 1747 to 1749, published a "Short Account ..." (see Larsen, 1939) of the plants he had collected in North America, seeds of which he had taken back for culture in Swedish gardens. In both this publication and in his Travels into North America (published in Swedish in the years 1753 to 1761), appearing in a first English edition in 1770), Kalm mentioned ginseng (Panax quinquefolium L.) and noted that it also grew in China and Korea. Kalm also noted the reputed medicinal value attached to the plant by the Chinese and recounted that the French collected the plant in Canada for export (via France) to China.

The dissertation defended by Halenius was reprinted in 1751 and again in 1787 (Graham, 1966); this last edition utilized Latin binomials and appeared three years after Thunberg's Flora Japonica (1784) had been published. Thunberg, also a student of Linnaeus but by 1784 an established botanist and explorer in his own right, also made brief mention in the Preface to his flora that many Japanese plants also occurred in Europe, America, and the East Indies, but particularly in the northern and adjacent, vast Chinese region. Among the plants treated are several that give direct evidence that Thunberg considered some plants in Japan and North America to be the same. Included in this group are: [3]

Sanicula canadensis L.

Viburnum dentatum L.

Sambucus canadensis L.

Lilium canadensis L.

Lilium philadelphicum L.

Aesculus pavia L.

Saururus cernuus L.

Asarum canadense L.

Asarum virginicum L.

Juglans nigra L.

Arum dracontium L.

Arum triphyllum L.

Magnolia glauca L.

Clematis virginiana L.

Teucrium virginiana L.

Bignonia catalpa L.

Juniperus virginiana L.

Pinus strobus L.

Amorpha fruticosa L.

Adiantum pedatum L.

No comments were made by Thunberg in the text mentioning that these species also occurred in the American flora, but all had originally been described from North America. In three instances he provided keys to separate Japanese from American species. Thus Acer dissectum Thunb. is distinguished from A. negundo L., A. trifidum Thunb. is separated from A. pensylvanicum L., and Spiraea callosa Thunb. is contrasted with S. tomentosa L.

There is no evidence that Thunberg was aware of the Halenius dissertation, but the next botanist to comment on the floristic affinities between the two regions was not only aware of Thunberg's Flora Japonica but compared it with Gronovius's (1739, 1743) Flora Virginica. Luigi Castiglioni used a comparison of these two floras as evidence of the similarity of the climates of the two regions in his two volume work, Viaggio negli Stati Uniti, that was published in Milan in

[3] The nomenclature and taxonomy follows Thunberg. No attempt has been made to give modem names that apply to these plants in both regions today, but many are obvious.

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1790, three years after his return to Italy from the United States, where he had spent three years (1785-1787) traveling. In the account of his travels in the United States, he noted the affinities of many American plants, which he had observed growing, with those of Japan. In the second volume (p. 156) he wrote, "It is moreover to be observed that the plants are very similar to those of the eastern coast of northern Asia, the Magnolia, Illicium, Calycanthus, Ginseng and many others, for example, growing here as in Japan and China, thus confirming the assertion of Franklin that the eastern coasts of the old and new continents bear much analogy in their climates." The second volume concluded, moreover, with a lengthy section (pp. 169-402) entitled "Osservazioni sui Vegetabili piu utili degli Stati Uniti" in which numerous North American species were described with notes on their distribution and uses.

Castiglioni's work appeared in a German translation in 1793, and the original Italian edition was apparently reissued in 1845 (Stafleu & Cowan, 1976). It remained, however, for C. E. Faxon (1891) to point out the significance of Castiglioni's observations in relation to the biogeographical relationships between Asia and North America. Faxon's appreciation of Castiglioni's work also remained relatively unknown until Li (1955) discovered it, after his 1952 survey of the history of the recognition of the relationships had been published.

Frederick Pursh

 Frederick Pursh (1814) was apparently the next botanist to comment on eastern North American - eastern Asian floristic relationships. Pursh's contribution to the knowledge of plant disjunctions between eastern North America and eastern Asia, however, was relatively slight and primarily limited to the single sentence describing the Pallas herbarium, which was quoted by Constance (1972) and is repeated here: "This extensive collection, now in the possession of A. B. Lambert, Esq., was highly useful to me, in comparing the plants of North America with those of the north of Asia, to some of which they have great affinity, and others are common to both continents." The few other instances where Pursh made note of eastern Asian - eastern North American disjuncts in his Flora are indicated in Appendix 1.

Thomas Nuttall

 In twentieth century reviews of early botanical literature dealing with the disjunct distribution of plants between eastern Asia and eastern North America, Asa Gray has generally been given the major share of the credit for being among the first to point out this pattern (Fernald, 1931; Hara, 1952, 1972; Li, 1952; Graham, 1972; Good, 1974). Because of the detail of Gray's (1840, 1846, 1865, 1857, 1859) studies, the attention they received at the time, and their contemporary importance in relating to Darwin's ideas on evolution, Gray has also been credited with being among the founders of the field of biogeography (Good, 1955; Li, 1952). The few papers that preceded Gray's (Halenius, 1750; Thunberg, 1784; Castiglioni, 1790; Pursh, 1814) were in no way as detailed, nor as critical in their analysis of this pattern of disjunction as was his "Diagnostic Characters" paper of 1859. Each of these papers tended to point out that some of the plants from North America were the same as plants in distantly separated parts of the world, particularly those in eastern Asia. None of them discussed the implications of populations of a single species being so widely separated and, with botany in most of the Northern Hemisphere still in the exploratory stage and, with vast areas still botanically unknown, it is unlikely that they should have. The explorations that continued through the early part of the 1800s in the American West, in eastern Asia, in Canada, and in other parts of the world led to the discovery of large numbers of new species and made available numerous specimens to help fill in gaps in plant distributions. The floristic works mentioned above that were based on these explorations took into account specific geographical regions and were often based on very limited numbers of specimens so that it was difficult to make direct comparisons with other parts of the world except through the literature. Even so, the influx of specimens contributed to the growth of the large private herbaria that were acquiring specimens at the same time and eventually made possible the kinds of comparisons of the world's flora made by Gray.

One exceptional work, however, that preceded Gray's papers, and one that has apparently been overlooked despite its importance to North American taxonomy, is Thomas Nuttall's (1818) Genera of North American Plants. Nuttall's Genera was the culmination of nearly ten years' study of the plants of North America, particularly those

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of the area that was then the United States and its territories. Nuttall was unusually well prepared to write the Genera for he was equally familiar with many North American plants in the field and in the herbarium, and had become familiar with others that were growing in the gardens of Philadelphia, particularly those in Bartram's Garden (Graustein, 1967). His studies before writing the Genera included field work across much of the northern United States from the East Coast nearly to the Rocky Mountains, travel down the Mississippi River to New Orleans, field work in the eastern coastal region, especially around Philadelphia, short excursions into New York state and into Canada around Niagara Falls, and a fairly long trip through the southeastern United States that retraced parts of the routes of several earlier botanical explorers including those of Bartram and Michaux (Graustein, 1967; Pennell, 1936). From the time of his arrival in Philadelphia in the spring of 1808 Nuttall had ready access to the largest herbaria in the United States through his connection with Benjamin Smith Barton and other Philadelphia botanists and was later able to study specimens in the large private herbaria in England before preparing his Genera.

Nuttall was also apparently aware of the works of earlier and contemporary botanists. In the introduction to the Genera he stated that he relied heavily on Pursh's (1814) Flora of North America, for which he considered his Genera as "supplementary"; Sprengel's (1802) Introduction to the Study of Botany, whose style he followed for the description of genera; and on Jussieu's (1789) Genera Plantarum, whose inclusion of notes on habitat for each genus he followed. Although Nuttall argued strongly for a classification in which genera were arranged according to their natural affinities, he followed the Linnaean sexual system, which was in use in the United States at the same time, although he did comment on the natural relationships of several genera. Nuttall was apparently able to determine the worldwide distribution of North American genera during a stay in England after his long collecting expedition up the Missouri River with the Astorians in 1811. He had planned only a short visit to England after his trip to the American West in 1810 and 1811 (Graustein, 1967), but it was more than two years before he could return to the United States for he was caught in England when the War of 1812 broke out. He made good use of his time by contacting the leading botanists in England and in examining the holdings of Joseph Banks's and A. B. Lambert's herbaria; the latter herbarium contained specimens collected by Pallas in Siberia and eastern Asia. Nuttall's familiarity with the plants of the northern United States and the chance to examine specimens in the Lambert and other herbaria, where he no doubt saw many North American plants that were still unknown to him, prepared him well for a trip to the southeastern United States when he returned to Philadelphia in 1815. He was able to retrace the routes of many of the first botanical collectors to that part of the country as he traveled through the mountains of Virginia, Kentucky, Tennessee, and the Carolinas and across the Piedmont and Atlantic Coastal Plain provinces of the Carolinas and Georgia (Graustein, 1967). Nuttall was also able to examine specimens in the herbaria of several noted contemporaries who lived in that part of the country and saw additional plants from the southeastern United States in their collections. One particularly useful collection was that of William Baldwin, who had collected specimens in western Florida during two years spent there while in the Navy (Graustein, 1967). Following these field and herbarium studies, Nuttall spent most of the years 1816 and 1817 writing the Genera, which was published in 1818 (Pennell, 1936).

The most important aspect of the Genera from a phytogeographical standpoint was Nuttall's inclusion of comments on "A view of the geographic distribution of each genus ... , not always sufficiently accurate for the existing state of the science, and the rapid progress of modern discovery" (Nuttall, 1818, p. vii). These "views" on geographic distribution were Nuttall's attempt to explain the relationships and distribution of North American plants on a worldwide scale.

Nuttall mentioned the geographical range and made other comments for each genus he treated. While he did not limit his observations to plants exhibiting an eastern Asian - eastern North American distribution, the comments of interest to us here are those that pertain to that pattern and show that Nuttall was aware of a phytogeographical connection between the two regions. In some cases Nuttall merely stated that a particular North American genus was also found in eastern Asia (here taken to include the Indian subcontinent and the Himalayas), but in other cases it seems clear that he was well aware of the significance of disjunction between the two widely sep-

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arated regions. Nuttall's comments (pp. 274, 275) under Chimaphila, a genus in which he recognized two species, C. umbellata and C. maculata, are especially insightful: "Indigenous also to the North West Coast of America. - Menzies. Probably both species of the genus will be found also in East Asia and Europe." The prediction that plants peculiar to one of these regions might be expected in the other was later made by Gray (1859, p. 416) and discussed by Wood (1972), but the fact that Nuttall had earlier suggested this has apparently been overlooked.

Although Nuttall did not restrict his comments to plants in those two geographical regions, we have listed in Appendix 1 examples from the Genera where he indicated disjunction between eastern North America and eastern Asia. The few instances where Pursh (1814) had previously commented on the same genera are also indicated. It must be remembered that taxonomic concepts of the early 1800s were different from our own. Nevertheless, Nuttall's ideas on phytogeography and the relationships of the North American flora were remarkably modern in many respects and deserve to be accorded their rightful place in the progression of knowledge of plant geography. Nuttall has yet to receive the credit he is due for the phytogeographical significance of his work, and none of his many biographers (Anonymous, 1860; Beidleman, 1960; Durand, 1860; Ewan, 1971; Graustein, 1967; Jones, 1937; Pennell, 1936; Smith & Thieret, 1959; Stuckey, 1968) have mentioned it. We hope that by indicating these examples of his awareness and interest in plant distributions that future biogeographers will acknowledge particularly his contribution to the study of eastern North American - eastern Asian plant disjunctions.

Asa Gray

 As indicated above, Asa Gray was not the first botanist to point out the relationships of the eastern North American and eastern Asian floras, but Gray's series of papers on this topic did focus scientific attention on this disjunct distribution pattern. Moreover, Gray fully examined and analyzed the problem utilizing the materials available to him and in so doing laid the foundation on which future work was possible. Gray's hypothesis of the causes of the eastern North American - eastern Asian disjunctions, moreover, brought the observed pattern into the scientific realm of biogeographic discussion and analysis on a multidisciplinary level.

The first evidence that Gray was aware of the relationship appeared in his reviews (1840, 1846; see Graham, 1972a, b, and Stuckey, 1978, for reprintings) of Siebold and Zuccarini's Flora Japonica and Florae Japonicae Familiae Naturales, in both of which he listed genera and species that were either similar, or common in both eastern North America and Japan. Gray apparently put aside this topic for about ten years, since the next mention he made of the relationship in print was in 1856 in the preface to his list of determinations of specimens collected in Japan by S. W. Williams and J. Morrow. Both of these men had been attached to Perry's expedition to Japan and the China seas. In his introductory remarks, Gray again mentioned the floristic relationships and indicated that he hoped to comment on them again once Charles Wright's collection of Japanese plants had been studied and named. Wright had been one of the botanists on the North Pacific Exploring Expedition under the command of Commodore Rogers and had returned home in 1856, during the time that Gray was involved in the preparation of his "Statistics of the Flora of the Northern United States." It was in this paper (1856, 1857) that Gray laid the framework of comparison that he was to utilize again in his now classic paper (1859) based on Wright's collections.

It is of interest to note that Gray had previously received and reviewed (Gray, 1854) J. D. Hooker's dissertation concerning the New Zealand flora (Hooker, 1853), which had been published in 1853 as an "Introductory Essay" to his Flora Novae-Zelandiae. The format of that essay in comparing the distribution and affinities of New Zealand plants was basically the same that Gray followed in his "Statistics" and "Diagnostic Characters" papers. It is also of interest to note that in his introduction Gray mentions that he was "requested by an esteemed correspondent ... to exhibit, in a compendious and convenient form, the elements of the flora I was occupied with." The esteemed correspondent, was, without any doubt, Charles Darwin, with whom Gray had begun to correspond in 1855, [4] and whom he

[4] Darwin's letters to Gray are preserved in the Archives of the Gray Herbarium of Harvard University, but most have been published. Likewise, many ofGray's letters to Darwin, the originals of which are preserved in the Cambridge University Library, have also been published. See Historical Records Survey (1939); J. L. Gray (1893); and F. Darwin (1887, 1903).

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mentions elsewhere in the "Statistics" paper. From reading Darwin's letters to Gray during the period 1855 to 1859, and from studying and comparing Hooker's New Zealand essay with Gray's subsequent papers, one is given the distinct impression that Darwin's questions and suggestions prodded Gray into action, and that Darwin's and Hooker's ideas strongly influenced Gray's thinking concerning the geographic affinities of the North American flora and the analysis of these relationships.

In his response (22 May 1855) to Darwin's first letter, Gray wrote that he "... had already intended, when the Flora of N. America should be finished to work up the geographical and climatic relations of this flora, and to compare it critically with the N. European and N. Asiatic floras." He continued by stating that he welcomed Darwin's questions, "and, if you will kindly give me hints as to what is needed, and how to do it, I will undertake the comparison of the plants of this moderate area (bounded by the Atlantic Coast, New Brunswick, St. Laurence, Great Lakes, Mississippi, and Potomac or Chesapeake Bay) with the General N. Amer. Flora and with that of the northern part of the Old World, and print the result either in the volume itself [5], or in some Journal, as you suggest. So, you see, far from taking your suggestion amiss, I respond to it by asking you to tell me very particularly how to do it, so it may be of use."

In his second letter to Gray (8 June 1855) Darwin asked specifically concerning the geographical distribution of North American plants and their affinities and total ranges. Darwin wrote, "The ranges of plants, to the East and West, viz. Whether most are found in Greenland and Western Europe, or in E. Asia appear to me a very interesting point as tending to show whether the migration has been Eastward or Westward," and he expressed pleasure that Gray was thinking of "drawing up some notice on geographical distribution...."

In his third letter to Gray (dated 2 May, probably 1856?) Darwin wrote, "I would give a list of temperate plants, if any, found in Eastern Asia, China, and Japan, and not elsewhere. Nothing would give me a better idea of the Flora of U.S. than the proportion of the genera to all the genera, which are confined to America, and the proportion of genera confined to America and Eastern Asia with Japan; the remaining genera would be common to America and Europe and the rest of the world; I presume it would be impossible to show any especial affinity in genera ... between America and Western Europe; America might be related to Eastern Asia, (always excluding Arctic forms) by a genus having the same species confined to these two regions; or it might be related by the genus having different species, the genus itself not being found elsewhere."

After receiving "sheets" of the first section of Gray's "Statistics" paper, Darwin wrote (12 October 1856) that nothing had surprised him more than the greater generic and specific affinity between the eastern North American flora and the eastern Asian flora, an affinity greater than that between the eastern and western American floras, and he asked if climate explained this affinity or was it one of the "many utterly inexplicable problems" of botanical geography. In another, undated letter written in 1857, Darwin requested Gray to contribute further to geographical problems despite the fact that Gray complained of his incomplete knowledge of plant distribution. Darwin urged Gray to prepare reports as the work progressed, and wrote, "So do pray look at this side of the question, and let us have another paper or two like the last admirable ones."

While Gray did mention the relationships between eastern North America and eastern Asia in his "Statistics" paper, these relationships were not developed fully, undoubtedly reflecting the fact that those comparisons he did make were based on literature reports and only a relatively few specimens. Thus in a section dealing with widely dispersed species that "reappear in Japan, the Himalaya, or some part of northern Asia, but are not European," Gray listed only the following 13 species: Brasenia peltata Pursh, Sium lineare Michaux, Cryptotaenia canadensis (L.) DC., Heracleum lanatum Michaux, Osmorhiza longistylus (Torrey) DC., Aralia quinquefolia (L.) Decaisne & Planchon, Viburnum lantanoides Michaux, Monotropa uniflora L., Phryma leptostachya L., Smilacina trifolia (L.) Desf., Trillium erectum L. var., Camptosorus rhizophyllus (L.) Link, and Adiantum pedatum L. By 1859, however, Gray had had the opportunity to study and name the rich collections of Charles Wright from Japan, and the list of shared species between eastern North America and Japan alone included 134 species! Gray now had the speci-

[5] The volume referred to by Gray was the new, second edition of his Manual of Botany of the Northern United States on which he was then working.

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mens that would allow for the floristic comparison with eastern Asia that Darwin had requested in his second letter.

Gray's (1859) published analysis of Wright's Japanese collection took the form of an essay in which he compared the Japanese flora with the North American and other north temperate floras. In it Gray described only the new taxa from among Wright's collections (these proposed in footnotes), indicating (p. 377) that a detailed account of the entire collection was "... intended to form a part of Mr. Wright's general report upon the extensive and interesting botanical collections made by him during the whole cruise of the Expedition. As this report, and that of the important scientific results of this Expedition in various other departments, may not be published for some time, I am permitted and requested, by the Commander of the Expedition, -ever considerate of the interests of science, - briefly to make known the principal novelties which have been discovered in this field. The discoveries ... also relate to the detection of known species in a region where they were not known before, and to the identification and elucidation of many obscure species. It will be best, therefore, to take a cursory general notice of the more interesting plants of this collection, adding any remarks which it may be worth while now to make upon their geographical distribution of their characters, and appending the diagnoses of new genera and species in the form of foot-notes." [6]

It is both in the text of the essay (where he elucidated previously described species and communicated his observations on the similarities of Japanese and other north temperate plants) and in the discussion of his "Tabular View of the Distribution of Japanese Plants and Their Nearest Allies in the Northern Temperate Zone" (pp. 424-436) that the disjunct distributional pattern between eastern North America and eastern Asia (Japan) became vividly apparent. Some of Gray's comments and observations concerning this pattern and the floristic similarities are quoted here.

In the discussion of the Berberidaceae (pp. 380, 381) Gray wrote, "But perhaps the most interesting and most unexpected discovery of the expedition is that of two strictly Eastern North American species of this order, - each the sole representative of their genus, - viz. Caulophyllum thalictroides, and Diphylleia cymosa, of Michaux .... Supposing these two plants to be satisfactorily identified as to species, [7] are we to regard them as the descendants of a common stock, though now separated by one hundred and forty degrees of longitude? Or are we to suppose them independently originated in two such widely distant regions?" And in the discussion (p. 416) of his newly proposed genus Heloniopsis, Gray wrote, "It may be briefly described as a Helonias with a few flowers, a single and slender style surmounted by a depressed-capitate stigma, and the seeds appendaged only at the hilum. Two things are noteworthy respecting this plant: 1. Its conformity to the rule, if it may be so called, that peculiar Eastern North American types have their counterparts in Japan. For the original and only true Helonias - one of the rarest plants in the United States - is found only in a few localities in New Jersey, the adjacent parts of Pennsylvania and Delaware, and in Virginia." Numerous other examples could be quoted, some dealing with identical species, others concerning closely allied species, or, as in the last example above, closely allied genera.

In his "Tabular View," Gray listed about 580 Japanese species "which have particular relatives in other and distant parts of the northern temperate zone" (p. 422), and listed in parallel columns the corresponding taxa represented in the floras of Europe; Central and Northern Asia; Western North America; and Eastern North America. Identical species shared between the flora of Japan and another area(s) were set in italic type. In analyzing this data, the following summary was presented (p. 437):

The whole number of Japanese entries is about 580

The whole number of Asiatic entries is about 44; in italics 274

The whole number of European entries is about 282; in italics 214

The whole number of W. American entries is about 216; in italics 162

The whole number of E. American entries is about 356; in italics 223

[6] Gray's fears that the scientific results of the expedition might not be published were justified; the complete manuscript that he alluded to remains unpublished, but is preserved in the Archives of the Gray Herbarium of Harvard University.

[7] The Japanese and North American species of both Caulophyllum and Diphylleia were subsequently shown to be distinct.

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Based on this analysis Gray was able to conclude (p. 437) that "It is interesting to notice that, notwithstanding the comparative proximity of Japan to Western North America, fewer of its species are represented there than in far distant Europe. Also, - showing that this difference is not owing to the separation by an ocean, - that far more Japanese plants are represented in Eastern North America than in either ... If we regard the identical species only, in the several floras, the preponderance is equally against Western as compared with Eastern North America, but is more in favor of Europe. For the number of species in the Japanese column which likewise occur in Western North America, are about 120; in Eastern North America, 134; in Europe, 157." But, considering the overall similarity at all levels, Gray concluded that of the 580 Japanese entries, 48 percent have corresponding European representatives and 27 percent are identical species; the comparative figures for western North America were calculated as 37 percent for corresponding taxa and 20 percent for identical species, while for eastern North America, the percentage of corresponding taxa was an overwhelming 61, and that for identical taxa was 23.

In summarizing the floristic data that he had presented concerning the New Zealand flora and its affinities with the floras of Australia and Tasmania and temperate South America, Hooker (1853, p. xxxvi) wrote that "Enough is here given to show that many of the peculiarities of each of the three great areas of land in the southern latitudes are representative ones, effecting a botanical relationship as strong as that which prevails throughout the lands within the Arctic and Northern Temperate zones, and which is not to be accounted for by any theory of transport of variation, but which is agreeable to the hypothesis of all being members of a once more extensive flora, which has broken up by geological and climatic causes." It was with a quotation of this passage that Gray concluded his review of Hooker's essay in 1854, and in attempting to explain the disjunct occurrence of identical species and the otherwise strong floristic affinities between the Japanese and eastern North American floras, Gray built a similar argument to explain the observed facts.

Gray wrote (1859, p. 440), "... it will be almost impossible to avoid the conclusion, that there has been a peculiar intermingling of the Eastern American and Eastern Asian floras, which demands explanation." On page 444 he wrote, "The discovery of numerous closely related species thus divided between two widely separated districts might not, in the present state of our knowledge, suggest former continuity, migration, or interchange; but that of identical species peculiar to the two inevitably would." Gray suggested further (p. 442) that the interchange between "the temperate floras of the western part of the Old World and the New had taken place via Asia," an idea that he attributed to Bentham. Gray (1859) credited Bentham with pointing out that interchange between all of the Old World, including western Europe, and the New World had taken place via Asia. Gray thought the statement may have been made in conversation or in correspondence between the two for Gray did not believe these thoughts had been published. Gray cited as evidence to support this theory the fact that such large North American genera as Aster L., Solidago L., and Eupatorium L. (Compositae), Euphorbia L. (Euphorbiaceae) and Solanum L. (Solanaceae) "... are represented in eastern Asia by a small number of species, which gradually diminish or altogether disappear as we proceed westward toward the Atlantic limits of Europe, while the types peculiar to the extreme west of Europe (excluding of course the Arctic flora) are wholly deficient in America." Bentham (1858, p. 35), in his "Synopsis of the genus Clitoria," had indeed published his view that there had at one time been an "ancient continuity of territory between America and Asia, or at any rate with a climate, more meridional than would be effected by a junction through the chains of the Aleutian and Kurile Islands."

It should be recalled that Gray was writing just prior to the publication of Darwin's (1859) Origin of Species, although Gray was aware of Darwin and Wallace's (1858) joint presentation to the Linnaean Society and indicated his acceptance of their views (p. 443, in footnote). Moreover, Darwin and Gray had carried on a constant and frequent correspondence during the period prior to the publications of Gray's "Diagnostic Characters" (see Good, 1955). Darwin had tested many of his ideas on Gray, anxious for both more data to test his hypotheses and Gray's reactions based on his knowledge of the American flora. Considering the influence of both Darwin and Hooker, it is perhaps not surprising that Gray concluded his analysis of Wright's Japanese collections with an attempt to explain the revealed disjunctions

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utilizing paleobotanical evidence and geological and glacial history (as they were then understood) coupled with an "evolutionary" outlook that embraced "common descent". Thus, the concept of a disrupted, once more widely distributed North Temperate flora was proposed as a working hypothesis to explain the observed facts. Gray had not only succeeded in fully pointing out the floristic similarities between eastern North America and eastern Asia, but he had (with due credit to Darwin, Hooker, and his geologist friend and colleague, Professor J. D. Dana) also proposed the working hypothesis of the causes - a hypothesis that remains valid in many ways today.

Gray's hypothesis gave support to Darwin's theory of evolution but refuted the theories of the Harvard naturalist, Louis Agassiz (Gray, 1860). Dupree (1959) has devoted several pages to the conflict between Gray and Agassiz and to the debate that both Gray's "Diagnostic Characters" and Darwin's Origin excited. That Gray's paper and Gray's influence surrounding the acceptance of Darwin's theories in North America were important undoubtedly brought Gray's "Diagnostic Characters" paper to the attention of a wider scientific public than might otherwise have been expected. It is understandable, therefore, that Gray has generally been considered the first botanist to recognize the eastern Asian - eastern North American pattern of disjunction.

The awarding of the major share of the credit to Asa Gray in connection with the floristic relationships between eastern North America and eastern Asia were also undoubtedly the result of Gray's continued interest in this phytogeographical distribution pattern and the published addresses concerning this topic that he delivered after the appearance of his "Diagnostic Characters" paper in 1859. Thus, in 1873, in his address to the American Association for the Advancement of Science (Gray, 1873a), Gray extended his floristic comparisons to include a broader region of Asia (Northeastern Asia, Japan to the Altai and the Himalayas) and to compare the relationships of this area with western North America (Oregon and California). In this paper Gray also appended a list of species common to the United States and Europe, a list which was later reprinted in England (Gray, 1873b). And in 1878, in a lecture presented to the Harvard University Natural History Society entitled "Forest Geography and Archeology," Gray outlined the similarities and dissimilarities of the forests of the temperate zone and compared those of the "Japan-Manchurian" region with those of Europe and eastern and western North America. In an effort to explain the similarities and dissimilarities he again emphasized that current distributions were the direct result of past history.

European Contemporaries of Gray

F. A. W. Miquel

Soon after the publication of Gray's "Diagnostic Characters," several European botanists returned their attention to the problem of the disjunct distributional pattern described by Gray, the affinities of the floras involved, and the hypothesis offered by Gray to explain biogeographical data. F. A. W. Miquel (1867a, b) a Dutch botanist, published two papers in 1867; one in which he discussed the affinities of the Japanese flora with that of North America (Miquel, 1867a), the other on the origins and phytogeographical relationships of the Japanese flora (Miquel, 1867b). While Miquel accepted the distributional pattern described by Gray and did not question the geological evidence of a once more widespread temperate flora in the Northern Hemisphere, he was unwilling to accept the Darwinian concept of common descent to explain the presence of "analogous" species in Japan and eastern North America. In his paper on the relationships of the Japanese flora with that of North America, Miquel tabulated 27 woody species common to Japan and all of North America, and 76 herbaceous species, not including analogous or similar species pairs. A year later (1868) he considered 81 species to be shared between eastern Asia and all of North America. While he agreed to the single (vs. the multiple) origin of a species, he maintained that the analogous species in each area were "created" independently and were not descendants from a common stock.

Adolf Engler

 Aware of Darwin's, de Candolle's, Gray's, Hooker's, and Bentham's ideas and the work they had done in this area, Adolf Engler (1879) used evidence from a number of fields in his attempt to interpret the history of the world's vegetation since the Tertiary and to explain present-day distribution patterns. Engler was convinced that the distribution of extant plants was the result of disruptions in ranges, changes in climate, migration, and evolution, and he placed the greatest importance on past geological and climatic events as the causes. He believed that numerous extant

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species had histories that extended back to the Tertiary, and he was aware that during the Tertiary the vegetation of the eastern part of North America extended westward at least as far as the Rockies and had occurred there into the Neogene. Engler cited fossil evidence based on the work of Lesquereux to support this hypothesis. He noted that many eastern North American taxa that were not now extant in western North America were represented there by fossils, but that only three western genera. Prosopis L., Cassia L., and Ziziphus Miller, were known as fossils in the East. Parallel forms in eastern and western North America were considered to be remnants of a once more widespread floristic element. Engler postulated that the Rockies had been lower in the past, the prairies less extensive, and that a mostly continuous, or at least similar, flora extended from the East to the West Coast, and that this flora became disrupted through a drying of the climate in the West, but evidence of a once more widespread flora still remained in such taxa as Ptelea angustifolia Bentham, Aesculus californica Nuttall, Acer negundo L. (as Negundo californicum Torrey & Gray), Staphylea bolanderi A. Gray, Rhus diversiloba Torrey & Gray, Cercis occidentalis Torrey ex Gray, Amelanchier alnifolia Nuttall, Calycanthus occidentalis Hooker & Arnott, Rhododendron californicum Hooker, and Styrax californicum Torrey (Engler, p. 12) in the West. All of these plants have disjunct counterparts in eastern North America.

Based on the paleobotanical studies of Heer, Engler cited fossil evidence of relatives of extant plants of Japan occurring on Sakhalin, and believed that the climate of Japan in the Miocene had been similar to that of present day southeastern Asia and supported a similar vegetation. He also felt that the knowledge of the Pliocene was insufficient and that a better knowledge of that period of time was necessary before a complete understanding of the history of the eastern Asian - eastern North American disjuncts could be obtained. Engler also lamented the fact that so little was known about the high mountains of China in a phytogeographical sense, especially since he was aware of the Japan-Himalayan-North American connection. He postulated that a rich Tertiary flora probably spread southward in China along the mountains that stretched from the Amur River region, around the Gobi Desert, to the Himalayas.

He gave examples of plants (Liquidambar L., Platanus L., Ostrya Scopoli, Castanea Miller) known as fossils in Europe, and believed that these genera had been richer in numbers of species, and were much more widespread in the past, as were many other genera now restricted to the warmer parts of North America and eastern Asia. Engler believed that the plants now restricted to eastern Asia and eastern North America had a much wider range, were even circumboreal, during the Tertiary because of a more widespread, equitable climate that prevailed then.

Another important aspect of Engler's work was that he pointed out that many of the eastern Asian - eastern North American disjuncts considered by Gray to be "identical" were already being shown to be distinct. He noted that Miquel (1868) had reduced Gray's (1859) list of 134 "identical" species to 81 taxa from Japan and North America that Miquel considered to be the same. Grisebach (1872, pp. 520-522, 602, 603) as pointed out by Engler, further reduced Miquel's list to only two examples, "Elodea petiolata" and Carex rostrata Stokes, of plants restricted to Japan and the eastern United States (21 plants were automatically excluded from consideration by Grisebach (1872) because they also occurred in Canada, and 41 were excluded on the basis of their occurrence in western North America). Engler (1879, pp. 25, 26), however, provided a list of 140 plants he considered to be shared between eastern Asia and North America, indicating the geographical region where each grew in the two regions. He also pointed out that the vast majority were forest plants, and only very few were found in open habitats. There are some errors in distribution data in the list, for example, Coptis occidentalis Torrey & Gray and Mahonia aquifolium Nuttall are both credited to Atlantic North America while Melothria pendula L. is credited to Pacific North America. In general, however, the list is more complete and refined than Gray's (1859) and indicates that many plants that Gray had considered to be identical are actually distinct.

In explaining the history of the world's vegetation Engler (1879) made considerable use of fossil evidence, especially of fossil floras in the arctic regions. He believed that the land surface had been lower between Europe and central Asia during the Tertiary and that the sea was not as great, perhaps resulting in a more equitable climate that accounted for both a widespread mixed mesophytic forest in present day arctic situations and the opportunity for an exchange of species across a bridge in northeast Asia - northwest North

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America. Engler postulated that the plants now restricted to the two disjunct regions were once more widespread further north, "as the Arctic flora now is," and that they had migrated southward as the climate cooled. These plants mostly became extinct in western North America because of the markedly different climate there, but still occurred in Japan and eastern North America where the climates remained similar. Those plants that survived in the West underwent evolutionary change in response to different edaphic factors, thereby resulting in different species of "Negundo" Boehmer, Rhus L., Cercis L., Osmorhiza Rafinesque, and Platanus L. in the eastern and western United States. The less drastic climatic changes in Japan accounted for the relatively higher number of genera, particularly for the higher number of monotypic genera, surviving there. Engler was apparently the first to refer to extant floras whose members were abundantly represented by fossils from the Tertiary at high latitudes in the Northern Hemisphere as the "Arcto-tertiäre Element." The extant regions referred to by Engler as supporting this "Arcto-tertiäre Element" were eastern North America, eastern Asia (Himalaya, Japan, northern China), and south in the Andes. This reference to the Andes suggests that Engler recognized an eastern North American temperate element in the flora of Mexico and Central America. This is the first indication of an awareness of this additional pattern of disjunction, an awareness that has generally been credited to Watson (1891; see Graham, 1973).

The extension of the recognition of floristic affinities to a wider area of the Asian continent went hand in hand with the development of floristic knowledge of China and other areas of mainland Asia. Thus, by 1873 H. F. Hance, who had been occupied with Chinese plants for many years, was able to list 21 species of the Chinese flora that occurred or were represented by closely allied species in the Caucasus region of southern Europe and western Asia, a region now known to support a flora rich in examples of Arcto-Tertiary elements. The first installments of Forbes and Hemsley's "Enumeration of All the Plants from China ..." appeared in 1886 and 1887, and in the systematic treatments the Chinese and extra-territorial distribution was given for each taxon. While this series was not completed until 1905, the wide circulation of the parts as they were published stimulated further collecting activity in remote areas of the Chinese interior (Thiselton-Dyer, 1905). The valuable collections made by Augustine Henry in central China were acquired by and distributed from the herbarium at Kew as a direct result of this stimulation. The continued discovery of botanical novelties in interior China, many of considerable botanical and horticultural interest, also increased interest in the botanical exploration of the Chinese empire. It was essentially in response to a strong desire to introduce these newly discovered ornamental species into cultivation in England that the Veitch nursery firm sent E. H. Wilson to China on two expeditions. Wilson not only carried on the work begun by Henry (and the earlier French missionaries), but brought the Chinese flora to the attention of late nineteenth and early twentieth century botanists.

Diels and the Flora of Central China

 As mentioned above, the gaps in the flora of central China that had been noted by Engler (1879) began to be filled within a few years as the collections of French missionaries and other European botanists began to reach Europe. Enough new material had been amassed and new information made available since Engler's (1879) paper to enable Diels (1900-1901, 1905) to write a definitive work on "Die Flora von Central-China" in which he discussed the history of botanical exploration, the geography, ecology, vegetation, and the relationships of the flora, in addition to providing a list of the known taxa. The area treated by Diels took in nearly all of the present-day province of Sichuan as well as the adjoining parts of surrounding provinces. Coincidentally, this happens to be a region containing a large number of taxa with North American relatives. Diels's comparison of the flora of central China with those of other parts of the world showed that a greater number of taxa were shared with Japan to the east than with the much closer Himalayan region to the west, although the vegetation in central China and Japan was somewhat different (Diels, 1900-1901). Diels showed that the Asian distribution of the elements in the flora exhibiting disjunction between eastern Asia and North America could be categorized into four types: 1) genera with members reaching to Malesia; 2) genera reaching from the Himalayas to Japan; 3) genera reaching only to Japan; 4) genera restricted to central China and North America. Examples of each are: 1) Arisaema Martius, Aletris L., Magnolia L., Schi-

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sandra Michaux, Illicium L., Polygonum L., sections Cephalophilon Meisner in Wallich and Echinocaulon Meisner in Wallich, Vitis L., Parthenocissus Planchon, Gordonia Ellis, Nyssa L., Clethra L., and Saururaceae; 2) Phryma leptostachya L., Panax L., Adiantum pedatum L., and several others; 3) Caulophyllum Michaux, Hamamelis L., Stewartia L., Diphylleia Michaux, plus others; 4) Cypripedium arietinum R. Brown, Calycanthaceae, Liriodendron tulipifera L., Decumaria L., and Gymnocladus Lamarck. Diels clearly based his classification of eastern Asian - North American disjunctions on the distribution of the Asian members of this pattern without regard to the distribution of the North American counterparts.

Mention must also be made of the Forest Flora of Japan, a volume Charles Sargent (1894) wrote after his first-hand observation of the Japanese flora during his journey there in 1892. Continuing in the tradition of Asa Gray, his mentor and friend, Sargent accounted for the ligneous flora of Japan and compared the genera with those that occur in eastern North America, essentially updating Gray's "Forest Geography and Archeology." Charles Sargent should also be given credit as one of the first Americans who realized that the climatic and floristic similarities between eastern Asia and eastern North America would allow for the successful cultivation of numerous Asian species in eastern North America. As first director of the Arnold Arboretum of Harvard University, Sargent had been successful with the cultivation of plants raised from seeds received from Emil Bretschneider, who was based at the Russian consulate in Peking, and immediately determined that the floristic exploration of China would not only result in furthering botanical knowledge of eastern Asia, but would also result in new ornamental plants for cultivation in America. To this end, Sargent engaged E. H. Wilson to collect specimens, seeds, and living plants in "western" China.

The history of the botanical exploration of China (for which see, in particular, Bretschneider, 1898) is not within the scope of this paper, but Wilson's activities both for the Arnold Arboretum and earlier for the Veitch Nursery firm in England during the period 1899 to 1910 greatly increased knowledge of the Chinese flora and extended the knowledge of the eastern Asian - eastern North American distribution pattern. Thus, Sargent (1913) was able in an Introduction to Wilson's A Naturalist in Western China, to compare the ligneous flora of North America with that of China, again pointing out the similarities but also noting the differences between the floras and emphasizing the richness of the ligneous flora of China. Further comparisons, particularly of the ligneous flora, were subsequently made by H. H. Hu (1935, 1936) and were the first contributions to the elucidation of this pattern of disjunction to be made by a Chinese botanist.

Numerous botanists have made significant contributions to the elucidation of the eastern Asian - eastern North American disjunct pattern of plant distribution during the twentieth century, both in the form of careful taxonomic studies of taxa exhibiting this distribution pattern and in summaries or overviews of current knowledge. Most of these contributions are well known to botanists and phytogeographers and may are included in our bibliography. However, much pertinent information is hidden in monographs, revisions, and other works under titles that often do not indicate a bearing on this topic.

The very useful summaries published by Hara (1952, 1956), Li (1952, reprinted 1971 with additions), Wood (1971, 1972), Raven (1972), and Thorne (1972), as well as papers published in this symposium volume, should be consulted for current views and numerous additional references not included here. Moreover, the volume edited by Graham (1972a) contains a collection of pertinent articles based on symposia held in conjunction with the XI International Botanical Congress in 1969 and a special Japan-United States Cooperative Science Program, which was also convened in 1969. While knowledge concerning the classic eastern Asian - eastern North American disjunct pattern of distribution has increased tremendously since the Linnaean era, much careful taxonomic work remains to be done before the relationships can be more fully understood and assessed. Darwin wrote to Gray in 1857 that "Undoubtedly careful discrimination of species is the foundation of all good work," and Wood (1972, p. 122) concluded his important paper by stating that "Morphology ... is basic to the taxonomy of extant and extinct plants, and, consequently, to the study of disjunctions." It is perhaps appropriate to conclude this brief historical survey by reiterating these sentiments - our knowledge and understanding of the eastern North American - eastern Asian floristic connection will in large measure reflect and be determined by our knowledge of the systematics of the taxa involved.

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