Darwin Seminar

Darwin's Orgin of Species is not an easy book even for a specialist. My own example may not be a good example, but let me say this much: I had to work hard in order to understand the significance of the principle of divergence, which appear in the last part of chapter 4. As was already discussed in my "Principle of Divergence", this principle plays a very important role when Darwin tries to explain why a small difference between varieties of the same sapecies become larger and larger, and two such varieties eventually become two distinct species. However, what Darwin says of this principle seems sometimes inconsistent, or at least misleading, and many readers, especially those who read the text carefully, may easily be confused.

The Principle of Divergence

The principle is introduced as follows:

the more diversified the descendants from any one species becomes in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers. (Origin, 1st ed., 112)

And, Darwin introduces a large diagram in order to illustrate how this principle can be used for explaining the genesis of a new genus (containing several species) from a single species. This topic was already discussed in his Big Species Book (published only in 1975), but I believe Darwin's "polished" description in the Origin is worse than the original version. Why do I say this?

Three Principles

In chapter 4, three Principles appear: (1) the Principle of Natural Selection, (2) the Principle of Divergence, and (3) the Principle of Extinction. But you've got to be careful; which are independent, and which are derivative? I am going to say that Darwin was quite misleading as regards this question, especially as to the Principle of Divergence. No one will doubt that the Principle of Natural Selection is a fundamental principle, not derivable from other two. Thus the question boils down to: Is (2) or (3) derivable from (1), or is it an independent principle?

Let us see what Darwin says.

Now let us see how this principle of great benefit being derived from divergence of character, combined with the principles of natural selection and of extinction, will tend to act. (Origin, 116)

Here, Darwin may seem to be suggesting that (1) alone is not sufficient, and we need two other principles. But is (2) on a par with (3)? A little reflection shows that this is not so, because Extinction (3) is nothing but a direct corollary of (1). Natural selection, in forming new species (which is more advantageous than some of the older species or varieties), inevitably causes extinction of some other species or varieties; thus the Principle of Extinction is not an independent principle. Then, what about Divergence, (2)?

Darwin says, many times, Divergence follows from Natural Selection

On this crucial question, Darwin says two contradictory things. At some places, he suggests that Divergence follows from Natural Selection. For example,

Owing to the divergent tendency of natural selection, the extreme amount of difference in character between species a^14 and z^14 will be much greater than that between the most different of the original eleven species. (Origin, 123, my italics)

Wow, does natural selection have already the divergent tendency? Then, why do we need Divergence as an extra principle? Compare the following passage where he refers to Extinction.

Within the same large group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in teh polity of Nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. (Origin, 125-6, my italics)

Yes, this is extinction, a by-product of natural selection! Then is Divergence also another by-product? Without giving a clear answer to this lingering question, Darwin now tries to summarize the whole chapter. And this summary magnifies our confusion.

In the Summary, Darwin again Repeats

Having said that natural selection accomplishes adaptation, Darwin continues:

But we already see how it [natural selection] entails extinction; and how largely extinction has acted in the world's history, geology plainly declares. Natural selection, also, leads to divergence of character; for more living beings can be supported on the same area the more they diverge in structure, habits, and consitution, ... (Origin, 127-8, my italics)

In this passage, Darwin treats (2) and (3) as if they are on a par. But then, why did he spend so much space for discussing the Principle of Divergence? But Darwin repeats the same point in the same page:

Natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life. (Origin, 128, my italics)

Again, he re-confirms, when he refers to the problem of classification, the same point a page later:

but, to the best of my judgment, it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram. (Origin, 129, my italics)

Thus, most readers may believe that Darwin's final position is that both (2) and (3) are derivative of (1)!

But All This is Wrong!

HOWEVER, your impression is all wrong! As I have analyzed some 10 years ago, the Principle of Divergence itself is an independent principle, not derivable from Natural Selection. And that's where Darwin's great originality lies, in comparison to Wallace's version of the theory of Natural Selection. Darwin himself seems to be misled by his own words; he confounded the Principle of Divergence itself, with the consequences from Divergence together with Natural Selection. Interested? Then why not read my paper (see Abstracts)?

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Last modified, December 10, 2002. (c) Soshichi Uchii

suchii@bun.kyoto-u.ac.jp