Encyclopedia of Evolution.pdf - Online Reading Center

434 appendix
same butterfly species! Clearly, the resemblance of the rare
to the abundant butterfly is an example of analogy, in which
the rare butterfly has evolved coloration and structure that
will allow it to, as it were, hide behind the protection of the
abundant species—sometimes so effectively that even scientists
could not at first tell the difference between the mockers
and the mocked; but the resemblances of the rare individuals
or species to one another are true homology of close evolutionary
relationship. “Now if a member of one of these persecuted
and rare groups were to assume a dress so like that
of a well-protected species that it continually deceived the
practiced eye of an entomologist, it would often deceive predaceous
birds and insects, and thus often escape destruction.
Mr. Bates may almost be said to have actually witnessed the
process” of evolution in action.
Finally, rudimentary organs speak clearly of evolutionary
ancestry. Hardly any animal can be named that does not possess
rudimentary organs, such as the functionless mammae of
male mammals, or the teeth that are present in but not used
by fetal whales and absent in the adults, which use plates of
baleen rather than teeth to obtain their food. Aquatic salamanders
have gills, but terrestrial ones do not—except the
unborn salamanders, which have very well-developed gills,
which they do not need. Most flowers have both male (stamens)
and female (pistils) parts, but some flowers have only
one or the other. It is common to find rudimentary, useless
pistils inside of male flowers. Some snapdragons have a rudimentary,
useless fifth stamen. Natural selection would get rid
of costly, unnecessary organs, but would not necessarily get
rid of them completely. When these organs have simplified
to the point of being rudimentary and at the same time no
longer costly, there has been no advantage in getting rid of
them completely. Rudimentary organs provide the same evidence
for the evolution of organisms as do silent letters for
the evolution of languages: The letters, retained in spelling,
are vestiges of ancestral pronunciations.
Why should rudimentary organs exist at all, if the Creator
made everything perfect? Some writers have remarked
that the Creator put them there “for the sake of symmetry”
or “to complete the scheme of Nature.” But this is, as
described earlier, not an explanation; nor is it even consistent
with itself: For why would the Creator have given rudimentary
[vestigial] hips to boa constrictors but not to other snakes?
… the several classes of facts which have been considered
in this chapter, seem to me to proclaim so plainly, that the
innumerable species, genera and families, with which this
world is peopled, are all descended … from common parents
… that I should without hesitation adopt this view,
even if it were unsupported by other facts …
chapter 15. recapitulation and conclusion
As this book represents one long argument, I will recapitulate
that argument, then draw some conclusions that emerge from
an evolutionary understanding of the world.
“That many and serious objections may be advanced
against … natural selection, I do not deny.” But even the
most complex adaptations can be explained from a few
simple starting premises, each supported by evidence. We
must be very cautious in claiming that any adaptation that
we observe could not have been produced by many small
steps, each selected by nature. All we have to do is look at
the tremendous diversity and advancements that have been
produced during the evolution of our crops and livestock.
“There is no reason why the principles which have acted so
efficiently under domestication should not have acted under
nature.” Natural selection seems inevitable. Even the slightest
individual differences can make the difference in natural
selection: “A grain in the balance may determine which
individuals shall live and which shall die.” Because of this,
“if there has been any variability under nature, it would be
an unaccountable fact if natural selection had not come into
play.” Further, “I can see no limit to” the power of natural
selection “in slowly and beautifully adapting each form to
the most complex relations of life,” so long as the heritable
variability is available in the populations. That it has done
so is indicated by the fact that we cannot distinguish species,
which many suppose to be independent creations, from
mere varieties, which everyone admits have a natural origin.
The fact that most groups become extinct and a few of them
diversify is inevitable, for if all species survived and diversified
“the world could not hold them,” and this also explains
why the larger groupings [phyla, orders, etc.] are so few in
number. It also explains why nature has so much variety but
so little innovation, as each lineage pursues its own course
of adaptation: “The same general end is gained through an
almost infinite diversity of means.”
This theory even explains why there is so much beauty in
nature—the beauty of birds and flowers is the result of sexual
selection or selection by pollinators. But it also explains why
not everything is beautiful, for the faces of “hideous bats
with a distorted resemblance to the human face” can evolve
as readily as something that we consider beautiful. We ought
not “to marvel if all the contrivances of nature be not, as far
as we can judge, absolutely perfect, as in the case even of the
human eye; or if some of them be abhorrent to our ideas of
fitness,” or appear to us wasteful, for natural selection produces
adaptations, not perfection.
This theory has an elegant simplicity. For example, the
occasional appearance of stripes on the legs of juvenile members
of the horse genus is simply explained as a vestige of
their common ancestors and is inexplicable on the basis of
independent creation of the species in that genus. Evolution
explains all the examples of characteristics that “bear the
plain stamp of inutility.” With elegant simplicity evolution
explains the patterns observed in the fossil record, both the
appearance and the permanent disappearance of species, and
the similarity of the species in any one layer to those in the
layers immediately above and below that layer. With elegant
simplicity this theory explains what every traveler has seen,
that the animals and plants are very different on the different
continents, even when conditions are nearly the same, and
that islands have relatively few species but those few species
are frequently unique.