In a letter written to his friend, botanist J.D. Hooker, in 1879, Charles Darwin wrote, “the rapid development as far as we can judge of all higher plants within recent geological times is an abominable mystery.” These words have commonly been understood to refer to the swift rise and diversification of the angiosperms during the Cretaceous period, a phenomenon that has puzzled botanists ever since both because of its speed and because there was little apparent precursor.
In a new article published in the American Journal of Botany, author Richard J.A. Buggs digs into botanists’ understanding of plant evolution and systematics in Darwin’s time to explore exactly what he meant by those words to Hooker, and why the problem might have been particularly vexing to him personally at that point in time.
When Darwin wrote his famous words to Hooker, there was no unified concept of flowering plants as a natural group. Monocots and dicots were considered to be entirely separate groups, and many considered dicots to be more closely related to gymnosperms. Furthermore, while dicot fossils had not been found prior to the Cretaceous, it was believed that monocots were much older, having co-occurred with gymnosperms during the Jurassic. So when Darwin wrote to his friend about “higher plants,” he almost certainly referred only to dicots.
While the rapid diversification of flowering plants has certainly been an ongoing problem for botanists, for Darwin, there was another dimension to the issue. Because the fossil record seemed to suggest very rapid change, this radiation event appeared to challenge the slow and gradual modification required by natural selection as a mechanism for evolution. Several of Darwin’s vocal critics were quick to take advantage of this discrepancy in their arguments against his theory. “In 1879, Darwin may have felt that the plant fossil record had been very publicly weaponized against him,” writes Buggs. “An abominable predicament indeed.”
Following Darwin’s death, a more modern understanding of the phylogenetic groupings of flowering plants emerged, with monocots now considered a part of the angiosperms (a term still not in regular use at that point). By the time Darwin’s now-famous words to Hooker were published in 1903, the natural assumption of the reader would be that he was speaking of both monocots and dicots, and the publisher chose not to contextualize further. It has been understood in this way ever since. “Even if Darwin had meant the dicots when he referred to the “higher plants”, he would surely have agreed that the mystery encompassed all angiosperms had he been alive in 1903,” writes Buggs.