The Essence of Darwinism and the Basis of Modern Orthodoxy 107
short-faced tumbler differs immensely in certain characters from the rock pigeon,
yet by comparing the several sub-breeds of these breeds, more especially those
brought from distant countries, we can make an almost perfect series between the
extremes of structure" (p. 27).
Darwin uses method two in a special and crucial way throughout the Origin.
Several of the most telling critiques against Darwin's style of evolution by
gradualistic continuity—best represented in Mivart's famous argument (1871)
about inviability of "incipient stages of useful structures" (see Chapter 11 for full
treatment)—held that insensibly graded passages between putative ancestors and
descendants could not even be conceptualized, much less documented. Charges of
inconceivability took several forms, each reducible to the claim that you can't get
from here to there, however well the beginning and end points may function.
Consider the two most prominent formulations: (1) Early stages (when
rudimentary) could provide no adaptive advantage, however valuable the final
product (2) Major functional changes cannot occur because intermediary stages
would fall into a never-never land of inviability, with the original (and essential)
function lost, and the new operation not yet established.
Darwin offered a twofold response to these arguments, both using this second
historical method of sequencing. He first presented theoretical arguments for the
conceivability, even the likelihood, of intermediary stages in supposed cases of
impossibility. He argued that early stages, too small to work in their eventual
manner, could have performed different functions at the outset, and been coopted
later for another style of life. (Incipient wings, originally used in thermoregulation,
became organs of flight when they evolved to sufficiently large size to provide
"fortuitous" aerodynamic benefits—see Kingsolver and Koehl, 1985, for an
experimental validation of this scenario, and Gould, 1991b, for general discussion).
As the misleadingly named principle of "pre-adaptation," this concept of functional
shift became an important principle in evolutionary theory (see Chapter 11).
Darwin writes, using a verbal intensifier rarely found in his prose: "In considering
transitions of organs, it is so important to bear in mind the probability of
conversion from one function to another" (p. 191).
As a response to charges of inviability for intermediary stages, Darwin
invoked the important principle of redundancy as a norm for organic structures and
functions. Most important functions can be performed by more than one organ; and
most organs work in more than one way. By coupling these two aspects of
redundancy, transitions in single organs can easily be conceived. An organ doesn't
mysteriously invent a new function, but usually intensifies and specializes a
previously minor use, while shedding an old primary operation. This previously
major function can then be lost because other organs continue to do the same
necessary job.
Ironically, we now recognize Darwin's favorite example of such redundancy
as not only incorrect, but truly backwards (Gould, 1989b)—the evolution of lungs
from swimbladders. (In fact, swimbladders evolved from lungs, see Liem, 1988).
Darwin ran his transition in the wrong way, but his argument for redundancy as the
key to viability for intermediary steps remains