As a professor of botany at Harvard University for several decades, Gray regularly visited, and corresponded with, many of the leading natural scientists of the era, including Charles Darwin, who held great regard for him.

Agreeing to teach for one year, with a break from August to December 1832, Gray had to cancel his plans for an expedition to Mexico, which at the time included what is now the southwestern United States.

After completing his teaching assignment in Utica on August 1, 1833, Gray became an assistant to Torrey at the College of Physicians and Surgeons in New York City.

[41] In mid-April 1839 he left Paris for Italy by way of southern France, then visiting Genoa, Rome, Florence, Venice, Bologna, Padua, and Trieste.

[43] Departing Austria, Gray went to Munich, Zürich, and Geneva, where he met the prominent botanist Augustin Pyramus de Candolle, who died in 1841.

Gray spent a year in Europe, leaving for America from Portsmouth, England, aboard the sailing ship Toronto on October 1, 1839, and arriving back in New York on November 4.

[18][24][46] The regents at the University of Michigan were so impressed by Gray's work in Europe, including his spending about $1,500 of his own money on specimen collections, that they granted him another year's salary that covered him until the summer of 1841.

[46] While he was in Paris at the Jardin des Plantes, Gray saw an unnamed dried specimen, collected by André Michaux, and named it Shortia galacifolia.

Gray then set down to work on the expedition's plant sheets at the estate of botanist George Bentham, whom he had met eleven years earlier, and then with William Henry Harvey in Ireland.

Gray spent only three weeks in London and Paris, and on the way back he read the newly published Géographie botanique raisonnée by Alphonse de Candolle.

The natural sciences had become highly specialized, yet this book synthesized them to explain living organisms within their environment and why plants were distributed the way they were, all upon a geologic scale.

[76] This contrasts with Linnaeus' artificial classification, which was designed for ease of use and focused on readily observed aspects of a plant, particularly the differences in the flowers.

[78] Gray was a leading opponent of the Scientific Lazzaroni, a group of mostly physical scientists who wanted American academia to mimic the autocratic academic structures of European universities.

In his view, science proved the unity of all man because all human races can interbreed and produce fertile offspring, i.e., all members of a species are connected genetically.

Even the Southern botanists who supported the North, such as Alvan Wentworth Chapman and Thomas Minott Peters, were soon forced to cease communications with Gray.

[87] Harvard was able to secure substantial new funding for its botanical programs in the years after the war, but Gray encountered significant trouble finding a suitable replacement for his professorial duties.

[91] In late 1872 Charles Sprague Sargent was appointed director of the Botanic Garden, the newly constructed Arnold Arboretum, and new botany-related buildings.

Also, late in 1872, Ignatius Sargent, Charles' father, and Horatio Hollis Hunnewell both agreed to donate $500 yearly each to support Gray so that he could devote "undivided attention" to completing Flora of North America.

[102] After turning over his non-research duties to Sargent, Goodale, Farlow, and Watson, Gray concentrated more on research and writing, especially on plant taxonomy, as well as lecturing around the country, largely promoting Darwinian ideas.

[107] Another German-American botanist, Ferdinand Lindheimer, collaborated with both Engelmann and Gray, focusing on collecting plants in Texas, hoping to find specimens with "no Latin names".

Gray's and Hooker's research was reported in their joint 1880 publication, "The Vegetation of the Rocky Mountain Region and a Comparison with that of Other Parts of the World," which appeared in volume six of Hayden's Bulletin of the United States Geological and Geophysical Survey of the Territories.

"[124] Before 1846, Gray had been firmly opposed to the idea of transmutation of species, in which simpler forms naturally become more complex over time, including via hybridization.

Since Darwin had nothing prepared, the reading included excerpts from his 1844 Essay and from a letter he had sent to Asa Gray in July 1857 outlining his theory on the origin of species.

[128] The correspondence with Gray was thus a key piece of evidence in establishing Darwin's intellectual priority with respect to the theory of evolution by natural selection.

Gray saw nature as filled with "unmistakable and irresistible indications of design" and argued that "God himself is the very last, irreducible causal factor and, hence, the source of all evolutionary change.

He collected together a number of his own writings to produce an influential book, Darwiniana (1876); these essays argued for a conciliation between Darwinian evolution and the tenets of theism, at a time when many on both sides perceived the two as mutually exclusive.

"[147] Darwin dedicated his 1877 book, The Different Forms of Flowers on Plants of the Same Species, to Gray, "as a small tribute of respect and affection".

[156] When the congregation moved into its present building in 1872, at 11 Garden Street, Gray planted two Kentucky yellowwood trees, Cladrastis kentukea, in front of the church.

An accompanying silver salver had the inscription "Bearing the greetings of one hundred and eighty botanists of North America to Asa Gray on his 75th birthday, Nov. 18, 1885.

"[13] Also received on his 75th birthday was a poem by James Russell Lowell: Just fate, prolong his life, well spent, Whose indefatigable hours Have been as gayly innocent, And fragrant as his flowers.