A Reappraisal of Charles Darwin’s Engagement with the Work of William Sharp Macleay

Charles Darwin, in his species notebooks, engaged seriously with the quinarian system of William Sharp Macleay. Much of the attention given to this engagement has focused on Darwin's attempt to explain, in a transmutationist framework, the intricate patterns that characterized the quinarian system. Here, I show that Darwin's attempt to explain these quinarian patterns primarily occurred before he had read any work by Macleay. By the time Darwin began reading Macleay's writings, he had already arrived at a skeptical view of the reality of these patterns. What most interested Darwin, as he read Macleay, was not the quinarian system itself. Rather, Darwin's notes on his reading primarily concerned certain background principles animating Macleay's work, in particular: (a) the non-existence of a saltus between human and animal minds, (b) the difficulty of establishing boundaries between species and varieties, and (c) Macleay's method of variation. Darwin's interest in the last of these left a mark on his discussion of taxonomic methodology in the Origin.