The Reluctant Mr. Darwin (19 page)

Taking an image from another of the old notebooks, Darwin writes: “The face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.” This is the struggle for existence. Something's gotta give. Just as not every wedge can fit, not every creature can find its place, meet its needs, achieving survival and reproductive success. Now consider, Darwin says at the start of
The Origin
's chapter IV, how this struggle interacts with the fact of variation.

Chapter IV is the book's core, in which he makes explicit his great analogy between domesticated varieties and wild species. If selective breeding by humans can create such peculiar modifications, Darwin asks, “what may not nature effect” through natural selection?

Imagine, say, an island filled with native creatures. Imagine that it undergoes a change of climate. The new climate presents new difficulties to the natives. Making matters worse for them, immigrant creatures begin invading across the water. “In such case, every slight modification, which in the course of ages chanced to arise, and which in any way favoured the individuals of any of the species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would thus have free scope for the work of improvement.” The “improvement” could take strange forms. Depending on circumstances, it might yield giant tortoises, pygmy deer, huge flightless birds, arboreal kangaroos, iguanas that dive for seaweed, mammoth cockroaches, or seed-cracking finches with the oral equipment of grosbeaks.

Furthermore, Darwin argues, natural selection leads to more than just small changes and nifty adaptations. It leads also to widening divergence between groups of creatures—between varieties, between species, between genera and higher categories—and thereby to the enormous diversity of life on Earth. That diversification is what allows such an abundance of individual creatures, and of different
kinds
of creatures, to co-exist within a small patch of forest, on an island, or in a little pond. As an instance, he mentions a single patch of turf, just three feet by four feet, that he examined himself. This patch had stood exposed and undisturbed for many years. Taking a complete census of the plants, Darwin found twenty species, representing eighteen different genera within eight orders. How could they all survive on such a small rectangle of dirt? They survived because they differed sufficiently from one another—in the ways they sought light, water, mineral nutrients, and reproductive success—and those differences minimized competition. Divergence is the phenomenon that allows greater amounts of biological success to be extracted from a finite amount of physical resources.

Given the Malthusian scramble for those resources, Darwin says,

With that sentence, especially its phrase about “places in the polity of nature,” written before the word “ecology” even existed, he prefigures the concept of ecological niche.

The book's middle chapters are devoted to complicated topics such as instinctual behavior, hybrid sterility (which helps preserve differences between populations once they arise), and Darwin's solution to the apparent problem of transitional structures. The last of those is an old objection against his theory, still sometimes raised by creationists. What he means by a transitional structure is one not fully developed to its higher potential—for instance, a slightly winglike appendage that's useless for flying, or the primitive precursor of an eye. The problem lies in understanding how natural selection could produce such half-baked pastries. If variations occur in tiny increments, and natural selection preserves only the advantageous ones, then what conceivable advantage adheres to each incremental change during the transitional phase—when a proto-wing is not yet aerodynamic, and a proto-eye cannot yet focus an image?

Darwin answers with examples of transitional forms that
are
adaptive (for instance, the “wings” on a flying squirrel or a flying fish) and with careful logic about the value of light-sensitive organs such as the ocelli in some invertebrates, which aren't fully developed eyes. He also suggests that the
type
of advantage may change fortuitously, from one opportunistic use to another, as a structure evolves. As illustration, he describes an obscure pair of structures known as the
ovigerous frena
, found among some barnacle species (the stalked ones) as two tiny folds of skin. These frena, these folds, secrete a sticky goo that helps hold gestating eggs within the body sack. The same basic organs appear in transmutated form among other barnacle species (the sessile ones), for whom they serve a different adaptive purpose, related to breathing. Lo, the two sticky folds have become branchiae. Darwin presents this case with sublimely erudite confidence, since he discovered and named the ovigerous frena himself. Who
says
the barnacle years were wasted?

34

By this time you're deep into the book. Only there, beginning with chapter IX, does Darwin shift his focus from the mechanism, natural selection, to the phenomenon, evolution. His tactical approach changes, too. Instead of arguing that natural selection
must
occur, he offers evidence that evolution
has
occurred. His evidence falls mainly within four categories: biogeography, paleontology, embryology, and morphology.

Biogeography, as you've seen, was the starting point of his own conversion to evolutionary thinking, and the guiding inspiration for Wallace, too. It's a vivid field of study, big and gaudy as all outdoors, but amid the sheer pageantry there's much buried meaning. Anyone who considers the geographical distribution of animals and plants, Darwin writes, must be struck by the clustering patterns among similar forms. Several species of zebra in Africa, none elsewhere. Several species of kangaroo in Australia and New Guinea, none elsewhere. Old World monkeys (the catarrhines) only east of the Atlantic Ocean, New World monkeys (the platyrrhines) only west of it. Many lemur species in Madagascar and its nearby islands; no lemurs anywhere else. Many toucan species in Central and South America; no toucans anywhere else. Where lemurs and toucans are absent, although habitat and climate might suit them, their roles are filled by other sorts of species—monkeys instead of lemurs in Africa, and hornbills there instead of toucans. Why? Are these patterns just accidental, or do they tell a story?

Darwin quotes Wallace's 1855 paper, the one whose meaning he missed on first reading, to the effect that “every species has come into existence coincident both in space and time with a pre-existing closely allied species.” Mr. Wallace and I now agree, he says, that this coincidence is explained by descent with modification, every species diverging from another in space and time. Adjacent areas of South America are inhabited by two similar species of large, flightless bird (the rheas), not by ostriches as in Africa or emus as in Australia. South America also has agoutis and bizcachas (largish rodents) in terrestrial habitats plus coypus and capybaras in the wetlands, rather than hares and rabbits in terrestrial habitats plus beavers and muskrats in the wetlands, like North America. Why not the same critters everywhere? Why should closely allied species inhabit neighboring areas on each continent? And why should similar habitat on different continents be occupied by species that aren't so closely allied? “We see in these facts some deep organic bond, prevailing throughout space and time,” Darwin says. “This bond, on my theory, is simply inheritance.” Similar species occur nearby in space because they have descended from common ancestors.

Paleontology reveals other clustering patterns, in the dimension of time. Look at the fossilized mammal bones and the living mammals of Australia, Darwin suggests. Look at the ancient and less ancient giant birds of New Zealand. Look at the fossil snails from Madeira and the species living there now. Are these similarities, between older and newer species, incomprehensible? Are they random happenstance? No. “On the theory of descent with modification,” he says, the occurrence of similar but not identical species during different geological eras within the same area “is at once explained.”

Embryology too involves patterns that beg for explanation. Why does the embryo of a mammal, passing through its developmental phases, spend one phase resembling the embryo of a reptile? Why does it, at another point, show gill slits like the embryo of a fish? And since embryology in a broad sense considers immature growth stages, not just unborn or unhatched forms, those questions lead to others. Why do lion whelps have striped legs, like the grown-ups of their close relative, the tiger? Why is a larval barnacle, swimming freely before metamorphosis, so similar to the larva of a brine shrimp? Why do the larvae of moths, flies, and beetles resemble one another (they're all wormy) more than any of them resembles its respective adult? Because, Darwin writes, the embryo is “the animal in its less modified state,” and that state “reveals the structure of its progenitor.”

Morphology, the study of anatomical shape and design, is in Darwin's words the “very soul” of natural history. What could be more suggestive than that the hand of a human (shaped for grasping), the paw of a mole (shaped for digging), the leg of a horse (shaped for running), the fin of a porpoise (swimming), and the wing of a bat (flying) should all reflect an underlying five digit pattern, with modified versions of the same bones in the same relative positions? Darwin doesn't claim to be the first naturalist to notice such homology; he reminds us that it was crucial to Geoffroy's formalist vision, which saw a “unity of plan” beneath the multiplicity of animal shapes. And the “same great law” of homologous parts can be recognized also in the mouths of insects. The long spiral proboscis of a moth, the folded snout of a bee, the fierce jaws of a beetle—they're made for different purposes but from common elements: mandibles, maxillae, an upper lip. The different parts of a flower, such as stamens and pistils, sepals and petals, are likewise homologous from species to species. What's the reason behind this recurrence of a few basic designs? Under the old view of special creations, Darwin notes, the only answer was “that it has so pleased the Creator to construct each animal and plant” with stingy economy of invention. That answer didn't really make sense. Why should an all-powerful deity economize? Darwin's answer is descent with modification. Homologies reflect the fact that natural selection—which isn't omnipotent and
is
economical, limited by history and circumstance—works upon patterns passed down from ancestral forms.

One use of morphology—the use to which Geoffroy and Cuvier put it—is systematic classification, the task of sorting all species into groups within other groups. Darwin, the former barnacle taxonomist, considers this topic worth twenty-three pages. The grouping of species into larger categories isn't arbitrary, he says, like sketching stars into constellations for human amusement or ease of memory; biological groupings are assumed to have some deeper basis. But what? Classifiers try to arrange species, genera, families, and other sets into a system that's not just handy for reference but somehow “natural” and objective. You can see those arrangements applied in the layout of any zoo. Here are the monkeys, there are the big cats, and in that building are the alligators and crocodiles. Birds in the aviary, fish in the aquarium. But what about porpoises and manatees? Clearly they're mammals, not fish, yet fishlike in their habitat needs—so do they go into the aquarium, too? Zoo designers may blur lines for the sake of practical convenience; taxonomists, on the other hand, do their best to recognize fundamental resemblances rather than superficial ones. For instance, all vertebrate animals (by definition of the category) have backbones. Among vertebrates, the mammals have fur and mammary glands, not feathers or scales. Among mammals, the marsupials have pouches in which they carry and nurse their dependent young. Among marsupials, the kangaroos have big feet and strong tails. What's the ultimate source of this orderliness? Many naturalists of his day, says Darwin, believe that a good system of classification simply “reveals the plan of the Creator.” That explanation doesn't satisfy him. It might or might not be true but it offers nothing in scientific knowledge. Darwin's alternative is that “all true classification is genealogical.” Commonality of descent, he says, is “the hidden bond which naturalists have been unconsciously seeking.” Alligators resemble crocodiles because they derive from common ancestors, not because of some divine choice to create multiple species of big aquatic reptiles with cone-shaped teeth.

Rudimentary organs are still another form of morphological evidence, illuminating to contemplate because they show that the living world is full of small imperfections: the blind eyes of a cave fish, the wing stubs of a kiwi, the appendix of a human. In a sense these are transitional structures too. But Darwin considers them separately from flying-fish wings, insect ocelli, and ovigerous frena in barnacles because rudimentary organs (he also calls them “atrophied” or “aborted”) seem to mark stages in a course of evolutionary deterioration (localized deterioration, that is, reducing the organ but not harming the creature overall) rather than stages in evolutionary improvement. If that isn't the explanation for their presence, and their odd uselessness, what is? For his readers Darwin raises the same question here, late in
The Origin
, that intrigued him privately back in notebook “B”: Why do men have nipples? And why do some snakes carry the rudiments of a pelvis and hind legs buried inside their sleek profiles? Why do certain species of flightless beetle have wings, sealed within wing covers that never open? Such vestigial features stand as remnants of the history of a lineage.