ASA GRAY. 755 



States that the existiuj? state of the science admitted of. He closes 

 with a general review of the characteristics of the North American 

 flora. 



In the course of the pages, he advocates the idea of a single area of 

 origin for a species, with dispersion at an epoch more or less ancient, 

 to account for distribution ; sustains Darwin's " surmise " as to the 

 species of large genera having a greater geographical area than those 

 of small genera; observes that a large percentage of the extra Euro- 

 pean types of eastern America are shared with eastern Asia, and 

 finds " that, curiously enough, eleven, or one-third, of our strictly alpine 

 species common to Europe — all but one of them arctic in the Old 

 World — are not known to cross the Arctic circle on this continent; so 

 that it seems almost certain that the interchange of alpine species 

 between us and Europe must have taken place in the direction of New- 

 foundland, Labrador, and Greenland, rather than through the polar 

 regions" (xxiii, 73). 



Two years later, in 1859, Dr. Gray had studied a collection of plants 

 from Japan (alluded to in the former paper, xxiii, 369, as iii hand), 

 which had been collected by Mr. Charles Wright; and his memoir on 

 the subject, read that year before the American Academy of Arts and 

 Sciences, closes with a sequel to the subject of Geographical Dis- 

 tribution, bringing out conclusions of still higher interest. He starts 

 off with the then new announcement and its evidence, that among the 

 plants of Japan, more species are represented in Europe than over the 

 nearer land, western i^orth America; more in eastern North America 

 than in either of the other two regions; and adds, that hence, there 

 has been a peculiar intermingling of the eastern American and eastern 

 Asia floras, which demands explanation. The explanation he finds in 

 the idea of migrations to and from the arctic regions, determined in 

 part, at least, by the climate of the preglacial, glacial, and postglacial 

 eras, and that the alpine plants of the'summits of the White Mount- 

 ains, Adirondacks, Black Mountains, and Alleghanies are species left 

 by the retreating glacier. 



Dr. Gray returned to this subject in his presidential address, in 1872, 

 before the American Association for the Advancement of Science,* 

 and, owing to the progress that had been made in the paleontology of 

 the continent, the arctic portion as well as the more southern, and de- 

 velopments elsewhere also, he was enabled to trace out the courses of 

 the migrations of plants, the Sequoias or Redwoods and many other 

 kinds, by positive facts with regard to the arctic and more southern 

 iioras ; and showed that the distribution southward into the western 

 United States, into eastern Europe or western Eurasia', and into Japan 

 and Asia or eastern Eurasia, was not only dependent, as he had before 

 put forth, on change in continental climates, but also that the particu- 

 lar direction southward was determined to a large extent bv fitness of 



•Am. Journal of Science, 1872 (3), iv, 282. 



