14 Michael Ruse
(remembering that many more individuals are born than can possibly
survive) that individuals having any advantage, however slight, over
others, would have the best chance of surviving and of procreating
their kind? On the other hand we may feel sure that any variation in
the least degree injurious would be rigidly destroyed. This preservation
of favourable variations and the rejection of injurious variations, I call
Natural Selection. (80-81)Here then was the master mechanism. There was more work to do in elabo-
ration. One important addition was the so-called Principle of Divergence: “from
the simple circumstance that the more diversified the descendants from any one
species become 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 na-
ture, and so be enabled to increase in numbers” (112). Darwin claimed to have
thought of this sometime in the early 1850s, although there are intimations in the
earliest species notebooks. For Darwin, the course of evolution was always one of
branching. This was built into the Galapagos discovery. A group splitting and
the parts going different ways, rather than all change being one of ancestor and
descendent without splitting. (Although he admitted some splitting, Lamarck’s
view of things was essentially that all change is from one form to another, in a
line.)
This key principle of divergence was one very much modeled on one of Darwin’s
favorite metaphors, namely the division of labor — something highlighted in the
economic world by the eighteenth-century, Scottish economist Adam Smith, and
a foundational principle of industrialists like Darwin’s grandfather Josiah Wedg-
wood. You get much more by breaking things up and specializing — intermediates,
the best at nothing, get wiped out quickly. In the organic world, Darwin saw adap-
tation functioning at its optimum when an organism stuck to its last and did what
it could best.
The advantage of diversification in the inhabitants of the same region
is, in fact, the same as that of the physiological division of labor in
the organs of the same individual body... No physiologist doubts
that a stomach by being adapted to digest vegetable matter alone,
or flesh alone, draws most nutriment from these substances. So in the
general economy of any land, the more widely and perfectly the animals
and plants are diversified for different habits of life, so will a greater
number of individuals be capable of there supporting themselves. A set
of animals, with their organization but little diversified, could hardly
compete with a set more perfectly diversified in structure. (115-116)Hence, divergence leading to (another famous metaphor) the tree of life.
The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species;