478 THE STRUCTURE OF EVOLUTIONARY THEORY
may almost be called living fossils; they have endured to the present day from
having inhabited a confined area, and from having thus been exposed to less severe
competition" (p. 107). In explaining why so few pairs of living species consist of
one highly modified descendant and one unchanged surviving ancestor, Darwin
invokes the high probability of substantial change, due to biotic competition, in
both lineages stemming from a common root:
It is just possible by my theory that one of two living forms might have
descended from the other; for instance, a horse from a tapir; and in this case
direct intermediate links will have existed between them. But such a case
would imply that one form had remained for a very long period unaltered,
whilst its descendants had undergone a vast amount of change; and the
principle of competition between organism and organism, between child
and parent, will render this a very rare event; for in all cases the new and
improved forms of life will tend to supplant the old and unimproved forms
(p. 281).Darwin's thought lies best revealed in a remarkable paragraph from the
Origin's final summary. All themes of this section now flow together—the denial
of mass extinction (as Darwin borrows Lyell's favorite rhetorical trick of conflating
this concept with nonscientific views of creation), the linkage of improvement in
some groups to the extermination of competitors, and the strongest statement in the
entire Origin about the predominant relative frequency of biotic competition vs.
response to altered physical conditions. For Darwin now makes the boldest
possible claim of all—an assertion that the ubiquity, continuity, and gradualism of
biotic competition might actually permit us to use morphological change as a
rough measure of elapsed time! *
As species are produced and exterminated by slowly acting and still
existing causes, and not by miraculous acts of creation and by catastrophes;
and as the most important of all causes for organic change is one which is
almost independent of altered and perhaps suddenly altered physical
conditions, namely, the mutual relation of organism to organism—the
improvement of one being entailing the improvement or the extermination
of others; it follows, that the amount of organic change in the fossils of
consecutive formations probably serves as a fair measure of the lapse of
actual time (pp. 487-488).
In summary, Darwin's link of progress to biotic competition completes his
argument against evolutionary systems (like Lamarck's) that propose separate
forces for progress and adaptation, and that, as an unintended result, fall
- Every time Darwin makes such an overextended statement, his own honesty and
subtlety draw him back immediately. The very next line presents the obvious caveat: "A
number of species, however, keeping in a body might remain for a long period
unchanged, whilst within the same period, several of these species, by migrating into new
countries and coming into competition with foreign associates, might become modified;
so that we must not overrate the accuracy of organic change as a measure of time" (p.
488).