246 Tim Lewens
Moving away from biology for a moment, it seems appropriate to offer something like
a typological explanation when we try to understand why some crystal structures are seen
frequently while others are not seen at all. We can take reference to “types” here to be
shorthand for sets of physical facts that make some crystalline forms stable, others unsta-
ble. Perhaps we can think of organic types in a similar way. The typologist claims that
only a few basic organic confi gurations are stable. These stable confi gurations then explain
the diversity of forms manifested by individual organisms.
Darwin says that species are formed from natural selection acting on slight variation.
His position demands then that these small variations, if they can be added up to produce
new species, are themselves stable. Hence the reason why we do not observe forms that
are intermediate between existing species cannot be that these forms are unstable. This
presents Darwin with a dilemma. On the face of things, if he is right about common
ancestry and the stability of slight variations, there should be no gaps between existing
organic forms. On the other hand, Darwin learns from typological thinkers such as Geof-
froy St. Hilaire and Owen that this is not the pattern we observe. So Darwin needs to give
a nontypological explanation for apparently typological phenomena.
Ron Amundson has argued persuasively that Darwin’s response is to reinterpret Owen’s
archetypes as ancestors: the diverse vertebrate species appear to be variations on a common
theme not because they are manifestations of a single timeless ground plan but because
they have retained the characteristics of a common ancestor (Darwin 1985 [1859]: 416;
see also Amundson [2005]: ch. 4). But Darwin’s way of thinking about shared history does
not guarantee that the world contains species that are what he calls “tolerably well-defi ned
objects” (Darwin 1985 [1859]: 210). We still need some explanation for the coherence of
species, and the gaps between them.
One of Darwin’s primary explanatory tools for discharging this task is an offshoot of
the more general principle of natural selection, which Darwin calls the “principle of diver-
gence of character.” Darwin had learned from Adam Smith that competition will be most
intense between individuals in the same line of business. Darwin argues that in the
economy of nature, no less than in human affairs, competitive advantage will come to
those who can open new markets, and fi nd new ways of making a living:
the more diversifi ed 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 diversifi ed places in the
polity of nature, and so be enabled to increase in numbers. (Darwin 1985 [1859]: 156)
Over time, generalists are squeezed out, and diverse specialists come to predominate. By
coupling principles such as this one to his hypothesis of common ancestry Darwin is able
to explain the existence of discrete species while also accounting for their underlying
commonalities, and he is able to do so in a nontypological way. This much is good news
for Mayr. What is not such good news for Mayr is that the primary resources Darwin uses
to replace typological explanation are natural selection and the “tree of life” hypothesis.