Under the light which these geological considerations throw upon the question, I cannot resist the conclusion, that the extant vegetable kingdom has a long and eventful history, and that the explanation of apparent anomalies in the geographical distribution of species may be found in the various and prolonged climatic or other physical vicissitudes to which they have been subject in earlier times: – that the occurrence of certain species, formerly supposed to be peculiar to North America, in a remote or antipodal region affords of itself no presumption that they were originated there; – and that the interchange of plants between Eastern North America and Eastern Asia is explicable upon the most natural and generally received hypothesis… (Diagnostic Characters of new species of phaenogamous plants collected in Japan by Charles Wright, 1858-9).
Perhaps no American scholar did more to advance Darwinian evolutionary theory in the United States than the Harvard botanist Asa Gray. He played a significant role in the U.S. publication of On the Origin of Species’ and reviewed the book for many major American journals (including The Atlantic). Behind the scenes, Gray’s correspondence was of critical importance to Darwin as he put pen to paper between 1855 and 1859. Darwin’s annotations of Gray’s letters bear testimony that the “facts” Gray collected were of great use to him in building his theory of species origin – Darwin often made notes of which chapters of his species book he would support with each letter’s “facts.”
Gray’s major theoretical contributions to botanical geography were, almost by definition, inextricable from questions of species origin. As early as 1840, Gray noted similarities between the floras of eastern Asia and eastern North America, despite their vast separation by a continent and an ocean. This phenomenon, which would become known as Gray’s “disjunction thesis” was a theme he would return to in his 1856 “Statistics of the Flora of the Northern United States” – a publication synthesizing the plant species distribution data that he had further developed at Darwin’s own request (Letter). Here, Gray hinted at the relationship between his work and Darwin’s, writing of its “bearings upon many questions of the highest scientific interest, respecting the geographical distribution, the mutual relations, the nature, and the origin of existing species of plants”.
Finally, in 1858, armed with new plant specimens from Japan, Gray was prepared to publicly elaborate, albeit briefly, on his own views on evolution. In his paper “Diagnostic Characters of new species of phaenogamous [seed] plants collected in Japan by Charles Wright,” read to the American Academy of Arts and Sciences in December 1858 and January 1859, Gray wrote that disjunct plant species raised “the question whether this actual geographical association of congeneric or other nearly related species is primordial, and therefore beyond all scientific explanation, or whether even this may be to a certain extent a natural result.” Citing Darwin and Wallace’s 1858 Linnaean Society papers, he wrote:
“But I am already disposed, on these and on other grounds, to admit that what are termed closely related species may in many cases be lineal descendants from a pristine stock, just as domesticated races are; or, in other words, that the limits of occasional variation in species (if by them we mean primordial forms) are wider than is generally supposed, and that derivative forms when segregated may be as constantly reproduced as their originals.” (443).
Thus was Gray’s own belief in evolution was briefly made public in a footnote of his paper on the disjuncts of eastern North America and eastern Asia in early 1859.
It would be difficult to assert that Gray promulgated a theory of evolution independent of Darwin’s own, for the two men were too closely linked by correspondence. After Darwin confided his theory of natural selection to Gray in 1857, Gray replied that his botanical experience led him to adhere very closely to the logic of the Darwinian framework:
“My notions about varieties are I believe just what you would have them…i.e. I believe every constitutional variety has a strong tendency to be perpetuated by seed, and the 2nd & 3rd generations a stronger tendency still to transmit their inherited peculiarities. And when we see that every plant man takes in hand develops into varieties with readiness, when favorably circumstanced, we cannot avoid suspecting they may do the same thing – i.e. sport in some way in the wild state also;- and that there is some law, some power inherent in plants generally prompting them to originate varieties. – which is just what you want to come to, and I suppose this is your starting point.” (August 1857)
In essence, Gray’s own data led him to Darwin’s doorstep, and his need as a scientist to seek basic laws of nature (and reject miraculous explanations) in regard to the biogeographical distribution of plant taxa launched him on the path to accepting an evolutionary explanation of biodiversity and becoming Darwin’s foremost advocate in North America.
