Asa Gray and the Discontinuous Distribution of Plants

In 1851 during one of his trips to Europe, Asa Gray (1810–88) visited Kew Gardens, where Joseph Dalton Hooker (1817–1911) introduced him to Charles Darwin. Hooker was director at Kew, and since 1842 Gray had been professor of natural history at Harvard University, where he devoted himself wholly to botany. He was the leading American botanist of the 19th century and an authority on plant distribution. The meeting must have gone well because from 1855 Gray and Darwin conducted a long correspondence, which began with a request from Darwin for information on the distribution of certain North American alpine plants. Gray supplied this, and Darwin made use of his knowledge of plant geography. In 1881 Darwin wrote to Gray that “there is hardly any one in the world whose approbation I value more highly than I do yours.” When, on June 18, 1858, Darwin received from Alfred Russel Wallace (1823–1913), another of his many correspondents, a paper entitled “On the Tendency of Varieties to Depart Indefinitely From the Original Type,” he realized that Wallace had developed an evolutionary theory very similar to his own, and therefore he had to publish his own theory. After consulting among his friends, Darwin decided that, with Wallace's agreement, he would prepare a statement of his own theory that would be read at a meeting of the Linnean Society together with Wallace's paper. Darwin compiled his paper partly from a letter written in 1857 in which he had outlined his theory to Asa Gray. The papers were duly presented at the meeting, but Darwin was too ill to attend and Wallace was in Asia, so Darwin's friends, Sir Charles Lyell (1797–1875) and Joseph Hooker, read them.

Gray did not agree with Darwin in every respect, but he was Darwin's principal champion in North America, and it was he who arranged for the U.S. publication of On the Origin of Species and negotiated the terms with the publisher, D. Appleton & Co. of New York. In 1888 Appleton published Gray's book Darwiniana: Essays and Reviews Pertaining to Darwinism, which proved very influential. Later, Gray, who was deeply religious, attempted to persuade Darwin to return to his Christian faith. Darwin appreciated the sentiment but was unable to accede. The two scientists met again in 1868, during a year's leave of absence Gray had taken from Harvard. The photograph at right shows him at about this time.

Asa Gray was primarily a plant taxonomist, but he also engaged in the other major botanical controversy of the 19th century: how to explain disjunct distribution—the occurrence of closely related species in scattered locations separated by substantial barriers to migration such as oceans. Many scientists believed that land bridges had once extended far from the shores of the continents, allowing plants to extend their ranges into regions that became isolated when the bridges disappeared. Gray had a different idea. He had identified plants growing in Japan that were closely related to plants found in eastern North America, but nowhere in between. He coined the term vicariance to describe the occurrence of two closely related species at widely separated locations, so that each species is the geographic equivalent of the other, and he did not believe that vicariance could arise through migration. Plants could not have crossed the Pacific Ocean and North American continent without leaving any traces of their journey. He suggested instead that these Japanese and eastern North American species had originated in the center of North America, from where they had extended their range in both directions until they became widespread. He produced a geological argument to show that conditions might have permitted this on at least two occasions in the past. Ice sheets had then advanced southward across North America, destroying all the plants in their path, but the ice did not extend to the eastern part of the continent, and the plants there survived. When the ice retreated, plants migrated northward to colonize the exposed ground, but the eastern plants remained unchanged. His idea influenced Darwin in the development of his own evolutionary theory.

Gray was born on November 18, 1810, at Sauquoit, New York. He studied medicine at Fairfield Medical School in Connecticut, qualifying in 1831, but he practiced medicine for only a few months. While a medical student, he taught himself botany in his spare time. In 1832 he began teaching science at Bartlett's High School, in Utica, New York, and in 1834 he became an assistant to John Torrey (1796–1873), professor of chemistry and botany at the College of Physicians and Surgeons in New York. Torrey was also a botanist, and he and Gray became lifelong friends and collaborators. Together they compiled the Flora of North America, published in two volumes in 1838 and 1843, a work that established them internationally as America's leading botanists. In 1835 Gray was appointed curator and librarian at the New York Lyceum of Natural History, and in 1838 he accepted the professorship of botany at the newly established University of Michigan. In that year he sailed to Europe to purchase books for the university and to study type specimens of North American plants in European herbaria. Gray spent a year in Europe, and then the opening of the university was delayed so he failed to take up the appointment.

In 1842 he accepted the post of professor of natural history at Harvard University on condition that he could devote himself to botany. Gray remained in that position until he retired in 1873. Gray was one of the first members of the National Academy of Sciences. He became president of the American Academy of Arts and Sciences and of the American Association for the Advancement of Science. He was a regent of the Smithsonian Institution. He wrote more than 360 books, monographs, and papers, one of the most famous being his Manual of the Botany of the Northern United States, from New England to Wisconsin and South to Ohio and Pennsylvania Inclusive, which was published in 1848. It is usually called Gray's Manual for short. Gray died at his home in Cambridge, Massachusetts, on January 30, 1888.