Darwin Seminar

The Editors' Introduction for the Chapter 6 begins with Darwin's recollection of the principle of divergence. Yes, most Darwin scholars now agree that that principle was a missing link between 1844 Essay and the Origin. The development of Darwin's theory, in this regard, was first traced by Dov Ospovat, based on the unpublished manuscripts in the "Black Box" at Cambridge University Library.

In the meantime, R. C. Stauffer edited and published in 1975 the portion of the Big Specied Book containing the crucial part where Darwin discusses Natural Selection together with the principle of divergence. It seems that Darwin was working hard for completing his discussion of the principle of divergence, when he received the paper by Wallace (Appendix 2), which was read as the "joint paper" at Linnean Society in July 1858. That's one of the reasons why several scholars suspect Darwin of some misconduct, as regards the "joint paper" and the completion of his idea of the principle of divergence. However, if you read carefully both Wallace's paper and Darwin's texts, it is quite clear that Darwin owes nothing to Wallace, as regards the principle of divergence.

However, there still remains some obscurity as regards the exact date of the arrival of Wallace's paper. Darwin's description in this regard is not reliable; he seems to be saying, intentionally, a wrong date. And the crucial document (Wallace's letter and its envelope on which we would find the stamp of the arrival date) was either lost or destroyed.

Setting aside these circumstances, the content of the Principle of Divergence is stated as follows:

...in any country, a far greater number of individuals descended from the same paretns can be supported, when greatly modified in different ways, in habits constitution & structure, so as to fill as many places, as possible, in the polity of nature, than when not at all or only slightly modified.

But how does this work? The role of the Principle of Divergence is suggested by Darwin as follows:

The complex action of these several principles, namely, natural selection, divergence & extinction, may be best ... illustrated by the following Diagram, ...

All right, the question now is: Is the Principle of Divergence distinct from that of Natural Selection?; what is the relationship between these two principles? But one thing is certain. Darwin is arguing that, with the Principle of Divergence, we can explain how the large differences between a species and another, between a genus and another, between an order and another, can be brought about--- how such an hierarchy is formed by evolution, via natural selection.

This principle of course appears again in the Origin (ch. 4), but it seems to me that Darwin's discussion there became more abstract, longer though, and it is one of the hardest parts for understanding. But his original discussion in the Big Species Book is sometimes more concrete, specific, and Darwin raises several specific questions, many of which are deleted in the Origin. Thus the Diagram (I) he mentioned can be understood with this image: We have 12 species of the same genus, A, ..., and M. A is the most moisture-loving plant and M the least moisture-loving plant.

Thus each species has a different character, peculiar to it and distinguishing it from any other species in the group. And Darwin's question is: what is going to happen if several of them continue to vary, competing with others? Variations are provided within each species; and there may well be the "places" in the polity of nature, which each varying species can occupy, provided that it can develop a character suitable for that occupation.

With such specific images, it may become easier to grasp the significance of Darwin's questions, such as:

I believe all the species of the same genus have descended from a common parent; & we may call the average amount of difference between the species, x; but if we look at the contemporaneous varieties of any one species, the amount of difference between them is comparatively extremely slight & may be called a. How thus can be the slight difference a may be augmented into the greater difference x; which must be on our theory be continually occurring in nature, if varieties are converted into good species?

...

But it may be objected that as natural selection, extinction & divergence must have been going on since the dawn of Life, why have we not an infinite number of species, almost as many species as individuals?

Darwin gave his own answers to these questions. Now, what do you think his answers are? Unless you read Darwin's text with this sort of reasoning and questioning back on your mind, you are wasting your time!

And here is a picture of Darwin's idea, as regards "how small differences become larger, and how a species diverges and gradually forms a genus or genera"; and this was the whole point of Darwin's discussion.

Thus, finally, we have 3 genera, and the ogirinal genus may now be regarded as a family or even an order!