 One way to see the complexity of Darwin’s own thinking on these issues is to follow the textual development of this concept between the close of the Notebook period (1839) and the publication of the Origin of Species in 1859. This period of approximately twenty years involved Darwin in a series of reflections that form successive strata in the final version of his theory of the evolution of species. Understanding the historical sequence of these developments also has significance for subsequent controversies over this concept and the different readings of the Origin as it went through successive editions, and as it was received in other cultural contexts (see below,  Section 3).  It also has some bearing on assessing Darwin’s relevance for more general philosophical questions, such as those surrounding the teleology of nature.