What can be more curious than that the hand
of a man, formed for grasping, that of a mole for digging, the leg
of the horse, the paddle of the porpoise, and the wing of the bat, should
all be constructed on the same pattern, and should include the same
bones, in the same relative positions?
Summary
Darwin discusses how the classification system was developed
by naturalists and is supported by his theory of descent with modification.
Naturalists have already developed complicated schemes for grouping
species, genera (groups of related species), and families (groups
of related genera), into arrangements that show the similarities
and physical relationships between existing species. Though Darwin
notes that the separation between family, genus, and species is
just as arbitrary as the separation between species and varieties,
he uses the rules by which scientists organize species into these
classifications to help bolster his theory of descent with modification. External
structures (those that relate to everyday habits or physical appearance)
turn out to be the least important way of classifying species, as
many unrelated species—such as whales and fish—have adapted similar
characteristics. These “adaptive” or “analogous” characteristics
appear in numerous animals and have the same physiological importance
(that is, they serve the same biological function from one to the
next) but cannot explain descent relationships, as the structures
were adapted by different species simultaneously rather than passed
down through heredity. Naturalists acknowledge that external appearance
is not basis enough to develop a classificatory scheme that would
explain which species are descended from the same parent species.
Instead, naturalists rely on internal structures, such as reproductive
systems and patterns in embryonic development, to classify species
into families and genera. Darwin uses these classifications to illustrate
chains of descent with modification.
One of the first places that naturalists look when determining classification
of species is morphology, which is the study of the underlying structural
relationships between physical organs of separate species. Several
species share similar bone and placement structures for physical
organs, even though these organs are shaped externally for each
animal’s particular “habits of life.” For example, a human being’s
hand shares a bone and placement structure with the front legs of
a mole, the paddle-shaped limbs of a porpoise, and the wings of
a bat. Natural selection explains these similar structures, in that
the physical adaptations seen in each species have been naturally
selected for that particular species’ use. The hands of a human
have been selected for grasping objects, the front legs of a mole
have been selected for digging, and the wings of a bat have been
selected for flying. This homology, or correspondence of bone structures,
is also seen within organs of the same individual organism. The
bones of the skull, for example, are homologous with the neighboring
vertebrae in the neck and spine, which suggests that the skull may
have developed as a modified structure from the neck of a former
species. These homologous structures show how organs developed within
a species and from one species to the next.
The development of embryos also illustrates how species
have descended from common ancestry. Darwin notes that many embryos
of different species appear initially to be similar, with species-specific
differences developing only as the embryo (or young) matures. The
development of the embryo mirrors the development of divergent characteristics
in each species. If a particular adaptation in a species appears
at a certain time in its development and causes its divergence from
the parent species, that adaptation will appear during the developmental
stage in its offspring. Darwin notes that this same phenomenon can
be observed in the offspring of humans. Infants will not take on
their inherited characteristics of eye or hair color, height, or
body type until they grow older. Since inherited modifications occur
later in life, the embryos of related species look alike at their
beginning stages and can therefore help naturalists determine relationships
of descent between species. Embryology, the study of embryos, allows
naturalists to observe the modifications that have caused species
divergences throughout time, providing clues about the arrangement
of species in classifications, which illustrate their descent with
modification from one another.
Also important for classification schemes are “rudimentary organs”—organs
that are left over from previous species and unimportant physiologically.
Sometimes, naturalists can easily tell which rudimentary organs
correspond to a past species. Leftover membranes on the backs of
insects, for example, clearly represent wings. Other rudimentary
organs are harder to classify, especially if an organ once used
for one purpose is now used for another, such as swim bladders in
fish that were once used for buoyancy but are now used for breathing.
These organs can reveal chains of descent, even if divergent species
remain completely unalike except for their one shared rudimentary
part. The existence of these rudimentary organs deals another blow
to natural theology, which claims that each species was independently,
perfectly created. Rudimentary organs reveal that the current state
of species is far from perfect: Useless organs have been rendered
such by disuse and therefore bolster the theory that species have
descended from one another.
Analysis
Darwin makes a brilliant argument by linking existing
classification systems to his scheme of descent with modification.
Naturalists had already recognized the structural and physiological
similarities between different species and had created sophisticated
ways of grouping species based on their similarities. Although naturalists had
developed intricate rules governing these classifications, they had
not explained why species should be grouped together in the first
place. Natural theology, which was the leading scientific explanation
for the origin of species at the time, argued that each species was
independently created and that any physical or physiological similarities
in species were either random or the brilliant design of their creator.
Darwin takes the existing classification system and uses it as proof
of his theory of descent with modification. Species can be classified
into genera and families, he says, because species descended from
one another, and the existing rules of classification provide patterns
that illustrate how this descent occurred.
By showing the imperfection of species, Darwin again argues against
natural theologians’ notion that species were created individually.
Natural theology hypothesizes that the perfect adaptations of organisms
to their environment is due to their independent creation by a creator.
Darwin points to the existence of rudimentary organs to expose inconsistencies
in this theory. If species were created to be perfectly suited to
their environment, he asks, why would their creator endow some species
with useless organs? The theory of natural selection, in contrast,
provides the perfect explanation for rudimentary organs. Only variations
advantageous to the species are naturally selected. But natural
selection weeds out only those variations or organs that cause the
species harm. It does not bother getting rid of unimportant variations
or organs. Advantageous variation causes the divergence of new species
but does not rid species of now-unusable organs from their predecessors.
Thus, rudimentary organs from predecessor species remain, rendering
species somewhat imperfect in structure. Although this argument
refutes natural theologians’ theory of the individual creation of
perfect species, it does not contradict Darwin’s notion that natural
selection creates perfectly adapted species. As noted in previous
chapters, natural selection does not automatically create perfect
species. Each new species becomes slightly more complex in its adaptability
to its environment, resulting in the seemingly perfect
species seen today. Natural selection explains both perfections
and imperfections in species in the natural world.
