Darwin in Baldwin Park

Charles Darwin (1809-1882) was training to be a parson when he received an invitation to travel the globe as the captain's gentleman companion aboard the HMS Beagle. For five years aboard ship and on travels across the land, Darwin collected and observed as the unofficial ship's naturalist. Once he returned to England in 1836, he never left southern England again.

Darwin retreated to a country home to raise a family and condense his evidence for his theory of evolution by natural selection. He published multiple large books, including On The Origin of Species in 1859 and The Descent of Man in 1871, along with smaller manuscripts, scientific papers, and over 9,000 letters to colleagues. He built a quarter-mile gravel trail, the Sandwalk, through an adjoining 1.5-acre wooded lot, and walked it daily to exercise and to think. He continued to make observations on what he saw in his garden and his Sandwalk, but limited his travels for the rest of his life. Every season brought something new, and he took to heart the later quote from Marcel Proust: "The real voyage of discovery consists, not in seeking new landscapes, but in having new eyes."

Matthias Baldwin Park is our neighborhood's Sandwalk, with something new to discover every day. This article will deviate from the topic of neighborhood history, and will discuss biology. This article is longer and more technical than our typical articles so that it can be a resource for teachers. The terminology will be a review for anyone who has ever taken a high school biology course. Science teachers in the area can use this page to jump start a field trip into the park for their students and can give the tour themselves, using this page and a 100-slide PowerPoint found here, or someone from the Friends group can lead the tour (contact baldwinparkphilly@gmail.com). Varied topics are presented here, so any teacher of biology should be able to find examples in the park of concepts discussed in class. Most of these are basic biology; some are probably more Advanced Placement Biology. There are links at the end of the article to other specific biology topics related to the trees.

A portion of Darwin's Sandwalk, a quarter-mile loop on a gravel trail through the woods

An aerial view of our neighborhood's Sandwalk equivalent, a one-eighth mile loop around the central raised beds

The Great Oxidation Event

I will use a chronologic narrative for this tour, starting with the dramatic stone plinths in the central landscaped beds designed by artist Athena Tacha.

The Earth is 4.5 billion years old, and for its first half- billion years it was a ball of molten rock. The Earth cooled, allowing the rocks to solidify and any water on Earth's surface to condense into the liquid state. The granite plinths placed as focal points in the beds are solidified molten rocks from more recent times, but are a reference to this early Earth. Life began on Earth as microorganisms, including cells that appeared three billion years ago that could use the carbon dioxide in the atmosphere and energy of the Sun to make their own food. A toxic (to them) byproduct of this photosynthesis was oxygen. For the first half-billion years of photosynthesis, the waste oxygen was chemically joined with the iron in the rocks to form iron oxides, or rust. It was only after all the iron was saturated with oxygen that free oxygen bubbled up into the atmosphere to make other forms of life possible.

An old rusty rail spike from the tracks in the Callowhill Cut displayed in front of a "rusty" granite plinth in the southeast section of the Park

Once the iron in the rocks was saturated with oxygen 2.5 billion years ago, the oxygen content in the atmosphere rose dramatically. The billion-year-long drop in oxygen between two and one billion years ago has not been fully explained, but during this time life on Earth was dominated by prokaryotes (basically cells without nuclear membranes -- the bacteria. Eukaryotic cells have a nuclear membrane and membrane-enclosed organelles). After the proposed snowball Earth event, when the entire Earth surface was glaciated, oxygen levels returned to their current levels. The diversification of multicellular life in the Cambrian explosion followed.

The First Organisms on Land

After the rocks on the surface of the Earth solidified, it would be a billion years before life emerged, and that happened first in the oceans. Single-celled algae and cyanobacteria (both capable of photosynthesis) then made their way onto solid land about 1.2 billion years ago. It was only about 500 million years ago that plants, fungi, and animals joined the terrestrial groups. Fungi and cyanobacteria could form symbiotic (mutually beneficial) relationships as tiny ecosystems called lichens. The fungus provides a foothold on the rock and the algae or cyanobacteria makes food for the pair via photosynthesis. The lichen also breaks down the rock surface, which along with the physical weathering of the rock, slowly forms a soil layer that allows other plants to move onto land.

lichen on plinth march closeup resize M filters.jpg

At least three different lichens competing for resources on a plinth in the park.
The same process of rock lichenification that occurred in Earth's early history repeats itself today when a volcanic island first raises its rocky surface above the ocean waves.

Photosynthesis and Symbiosis

The photo of the lichens above also illustrates the competitive exclusion principle: no two species competing for the same resources in the same niche can coexist. If one species has even the tiniest advantage, it will force the other species into extinction at that site. It will be interesting to follow the slow-growing lichens in the photo to see which species (and lichens have such a tight symbiosis that they are considered species even though comprised of two different species themselves) will win out.

The cyanobacteria are unicellular prokaryotes that use chlorophyll to manufacture food from carbon dioxide and water. About 2 billion years ago, a remarkable symbiosis developed. A cyanobacteria was ingested by a larger prokaryote but was not digested. The internal cyanobacteria and its larger predator lived much like the lichen: the cyanobacteria provided food via photosynthesis and the larger organism provided a safe environment. This was the origin of algae and true plants. In like fashion, a predator prokaryote ingested a smaller prey prokaryote that was capable of using oxygen to break down ingested food more efficiently. The smaller prey was not digested, and lived within the larger prokaryote, eventually developing into the mitochondria within most eukaryotic cells today.

Some early springtime mosses forming a fuzzy green layer atop the plinth on the left. Unlike lichens, mosses are true plants with tiny leaves. The mosses have no tubes to carry water (a vascular system) towards the top of the plant, so they are short and require moist environments. Mosses produce spores instead of seeds. These spores have a tough outer coat that allows them to endure dry conditions and start growing the next spring. Vascular plants like ferns and club mosses (none in the park) evolved about 420 million years ago, and these also produce spores.
The evergreen tree on the right is a white pine, a tree which does not surround its seed with a fruit. These trees are called gymnosperms ("naked seed"). Gymnosperms with seeds evolved about 360 million years ago, followed by angiosperms (plants with flowers and seeds encased in a fruit) about 140 million years ago.

The walls of the landscaped beds are built with 450 tons of argillite rocks from near Pottstown, Pennsylvania. Argillite is a sedimentary rock containing fossils, and the Pottstown formation has fossils of ferns and gymnosperms from 210 million years ago. This rock tops a wall on the southern section of the beds near the path.

Plants and Symbioses with Bacteria and Fungi

(Nitrogen fixation and Mycorrhizae)

There are other examples of symbioses in the park. Our page on the pagoda tree here talks about the relationship between nitrogen-fixing bacteria living in nodules on the roots of legumes like the redbud and honey locust trees. These bacteria convert atmospheric nitrogen into forms of nitrogen that the plants can use. The plants supply a home and carbon compounds to the bacteria. Another symbiosis example is that of the fungal filaments that live in conjunction with the roots of most plants. The fungus increases the surface area of the water and mineral absorbing roots, and the fungus extracts carbon compounds from the plant. This is discussed on our fungus page here.

Pea pod-like seed pods of a redbud tree in the park.
The roots of this and most other legumes (notably except for our pagoda trees) contain root nodules inhabited by nitrogen-fixing bacteria.

The seed pods of the pagoda tree, a legume without nitrogen-fixing bacteria. Biology defies strict classification and rules.

Evergreen versus Deciduous

Students often get confused with the multiple dichotomous classifications of trees. Are all evergreens conifers? Are all conifers evergreen?

Just outside the park in the northwest section are trees that can answer these questions. The bald cypress is a gymnosperm conifer that sheds its needles; it is a deciduous conifer (like dawn redwoods and larches). The hollies are broadleaf evergreens that are angiosperms, not gymnosperm conifers. The hollies flower and bear red fruit, but have a reproductive life that includes separate male and female trees. The two magnolias in the park, if existing in the southern United States, might also be evergreen, but at the latitude of Philadelphia they are deciduous.

Bald Cypress

American Holly

Bald cypress ("bald" because it is deciduous) and the evergreen flowering hollies just across the north fence.

Flowers

Angiosperms, plants with flowers and fruits, make up 90% of all plants today. Every plant you see in the park, except the conifers, is an angiosperm: the grasses, the perennial shrubs, and the trees other than the conifers. The flowers on the trees often go unnoticed, but if you look at our map of the trees in the park here, each species has a photo of its unique flower and fruit.

As noted above about the hollies, trees have different reproductive strategies to maximize the number of offspring and minimize the risk of self-fertilization. The dogwoods, hawthorns, and plum have flowers that include both male (stamen) and female (pistil) parts. The filbert and the sweetgum have separate male and female flowers, but all on the same tree (termed monoecious). The katsura goes one step further: each tree has either male or female flowers, but not both (termed dioecious).

Parts of an idealized complete flower, with concentric rings of modified leaves. From outside to inside these are sepals, petals, male stamens, and female pistils. The seed will develop within the ovary, which itself will develop into the fruit.

Plum flowers that have both male and female parts. The plum is one of the first trees to flower in the park, flowering before its pollinators, the bees, are active. Very few plums are produced without pollination.

The hawthorn has flowers that smell like rotting flesh, attracting flies to the stamens to carry off the pollen

Filbert with a hanging catkin composed of numerous tiny male flowers, with equally tiny female flowers budding just above the catkins.

The sweetgum has stiff vertical clusters of tiny male flowers, with the tiny female flowers forming a pendulous ball. The latter will become the spiky brown fruits that litter the ground below the sweetgum.

The tiny banana-shaped fruit on the female katsura in the west side of the park.
Only the female bears fruit. The male katsura in the south side of the park has flowers that are male, do not have ovaries, and do not produce fruit.

The gymnosperms do not have flowers or fruit, but they do have separate male and female cones. Some gymnosperms, like the gingko on the northeast corner of Hamilton and 20th Streets, are dioecious with separate male and female trees. The conifers in our park are all monoecious, with male and female cones on the same tree. The male cones produce copious pollen either in spring (black pine) or fall (cedar of Lebanon). The pollen will be borne by the wind to fertilize the female cones, which tend to be higher up, of other trees. These female cones will mature and release the winged seeds.

Clusters of black pine male cones (or pollen sacs) on April 15.
Often male cones will be lower on the tree, and female cones nearer to the top. These trees are wind pollinated, so having female cones higher in the tree lowers the chances of self-fertilization.

The female cones of the black pine are seen here.

In the Linnaean classification of organisms, an organism is assigned to a nested hierarchy of kingdom, phylum, class, order, family, genus, and species. In the park, the scientific names of the trees on the tree identification tags are included with genus and species. Each genus or higher level of classification would have a type specimen, meaning an exemplar of that level of taxonomy. The sweetbay magnolia, or Magnolia virginiana, is the type specimen for the genus Magnolia, which is the type genus for the family Magnoliaceae, which is the type family for the order Magnoliales, which is the type order for the clade (grouping like phylum) Angiosperms. So our little sweetbay magnolia represents the type specimen for all flowering plants, all quarter million species of them.

The unassuming but distinguished sweetbay magnolia, Magnolia virginiana

Monocotyledons versus Dicotyledons

Biologists classify, and this classification is used to show ancestral relationships. For example, an early split in the angiosperms formed two groups, the monocots and the eudicots. Most grade school children plant a seed in science class and draw pictures of the germinating plant over several days. The monocots have one seed leaf, whereas the dicots have two; a distinction usually drawn well by the young botanists. The grasses in the park, including the lawn, are monocots, as are the lilies. Two thirds of angiosperms are eudicots; accordingly, the trees and most of the plants in the landscaped beds are eudicots. Monocots are important to humans; we interact with monocots every day. Our food grains like wheat, corn, and rice are monocots.

Two large groups of angiosperms; monocots and eudicots.
The magnolias are an intermediate between monocots and eudicots and are therefore thought to have developed early in angiosperm evolution.

Vascular System (Circulation)

As mentioned, mosses don't have vascular systems to carry water to the top of the plant. The mosses are therefore short. Trees are tall, and need thin tubes to carry water to the leaves and to carry sugar to the roots and trunk. Dicotyledons also have the issue of adding not just height, but also extensive woody girth. A thicker woody trunk is necessary to support its height. Trees add girth by having a vascular cambium layer that adds xylem, which carries water upward to the leaves, in a ring to its interior; it adds a ring of phloem, which carries sugary liquid (sap) to the roots, to its exterior. This secondary phloem is a layer added each year, but sloughs off gradually with the bark.

If you cut across the trunk or a large branch, you can see the sap oozing from the secondary phloem, which appears to be coming from just within the outer bark.

Diagram of a cross-section of the trunk of a tree, showing the secondary phloem as part of the inner bark. As the diagram makes clear, most of the trunk of a tree is xylem (sapwood and heartwood). Each year's new growth of xylem constitutes a growth ring, with the inner part of that new growth less dense and more vascular. The secondary phloem is made each year as a new layer.

A recently cut branch on the cherry in the southeast section of the park, showing a ring of sap oozing out of the secondary phloem just interior to the cork cambium.

Why are Leaves Green?

Why are most trees, in fact most plants, green? Ask this of a random sample of people and you will get first a look of disbelief at the stupidity of the question, and then a look of puzzlement as to the answer. Almost all plants are autotrophs, meaning they can make their own food using carbon dioxide in the air, water in the soil, and the energy of sunlight. This sunlight comes in different wavelengths, which when combined give us the white light of the sun. When separated by a prism or by raindrops, sunlight gives us the separate colors of the rainbow.

This process of photosynthesis occurs in the leaves of a tree. The pigment in green leaves that is most prominent is chlorophyll. As the diagram below shows, the two chlorophylls, A and B, absorb mostly blue and red light and absorb little green light. This green light that is not absorbed is reflected, giving the green color.

The purple-leafed plum in the northwest section stands out

The photosynthesis pigments in leaves absorb blue and red light well. The green light is not absorbed but is reflected to our eyes.

There are accessory pigments called carotenoids that absorb some of the blue light that chlorophyll misses, which give them the colors yellow, red, or orange. These carotenoids give us the fall colors when the chlorophyll is broken down by the tree. Another pigment called anthocyanin is not involved in photosynthesis but gives stems, leaves, and fruits a red color. You would think that a red-leafed tree would be at a significant disadvantage in competition with green-leafed trees, but as it turns out, at high light levels there is not much of a difference in photosynthesis rates since the red-leafed trees do contain chlorophyll. At high light levels, the red anthocyanins may even offer some protection from harsh ultraviolet light. At low light levels, however, red-leafed trees are at a disadvantage. In fact, when purple leaf plum trees are planted in the shade, the leaves will be green rather than purple. This variation in color of the leaves is part of the plum's norm of reaction, that is, the different phenotypic expression of one genotype when in different environments (here, sun versus shade).

There is another reason red-leafed trees stick around despite a competitive disadvantage: humans like them! Humans select red-leafed trees as ornamentals and thereby ensure their continued spread. A more familiar analogy would be asking why poodles and other domestic dog breeds persist in a world where there are wolves. Humans select for dogs, and up until recently, even tried to exterminate wolves. In nature, however, competition between wolves and poodles would be a bit one-sided.

Carbon Fixation Mechanisms

Speaking of photosynthetic mechanisms, many AP Biology students will learn about the alternate paths used in plant leaves to maximize the conversion of carbon dioxide to sugar. As a refresher, plants make 6-carbon glucose molecules in two stages: the first stage is done in sunlight, whereby the energy of the sun is converted into the chemical energy in the molecule ATP. This ATP transfers usable energy around the cell, just as it does in our own cells. The second stage does not require sunlight, because the plant cells are using the ATP formed in the first stage to build a 6-carbon glucose molecule out of many 1-carbon carbon dioxide molecules. Oxygen obstructs this second stage and slows down the process. In addition, the stomata or small openings on the underside of leaves must be open to draw in CO2 and release O2, but this open state allows water to escape as well. So leaves have the tasks of concentrating the CO2 away from O2, and retaining water. Most plants use the C3 pathway, but some plants in hot dry environments have evolved either the C4 pathway or the crassulacean acid metabolism (CAM) pathway. The C4 pathway concentrates CO2 spatially in the plant cell, and CAM concentrates CO2 temporally by functioning only at night. Without getting lost in the weeds, so to speak, suffice it to say that the park has examples of all three metabolic pathways of carbon fixation.

For more on carbon fixation metabolism, see short outside article here.

Three mechanisms of carbon fixation, i.e. converting carbon dioxide in the atmosphere into carbon-containing starches and sugars.

Red and yellow sedum in the northeast section of the landscaped beds in May.
Sedum belongs to the family Crassulaceae, in which CAM was first discovered. Besides having thick waxy leaves, closing stomata during the day and using CAM also preserve water. The prominent yucca plants placed near the plinths are native to desert environments and also employ CAM. About 7% of all plants use CAM.

About 3% of plants use C4 metabolism. The asters in the raised beds are C4 plants. The pesky crabgrass and spurge growing in the park are two more, giving them an advantage over the lawn grasses which use C3. The paulownia trees (Paulownia tomentosa), seen here looking east from the 19th Street bridge, may also be rare C4-pathway trees. The paulownia have huge leaves and grow very fast.

Very large solar panel (leaf) from a paulownia tree just beyond the fence on the south edge of the park. Someday, some biology or physics student will grow up to invent an artificial leaf: a device that uses the energy of the sun to split the water molecule and converts the energy in ATP into the chemical energy in sugar.

Biogeography

In On the Origin of Species Darwin devoted two complete chapters to biogeography, how different life forms are adapted to different locations and how they got there. He was not aware of plate tectonics nor the effect of recurrent global glaciations on the rise and fall of sea levels. He still did pretty well. He would understand the change in plant and animal diversity during our current global climate change crisis. We can also see evidence of this change in the park.

The colors on the map below represent gradations of minimum average yearly temperatures in different parts of the United States. In just 16 years, the Philadelphia area has warmed from Zone 6 (light green) to Zone 7 (yellow). Plants like sweetgum and sweetbay magnolia are southern plants, but their ranges are extending northward. Likewise, sugar maples, a northern species, are retreating northward as the climate changes. Sugar maples are under stress in Philadelphia due to warming. Trees can live hundreds of years, so arborists are now taking these warming trends seriously when choosing plantings in an area. Sweetgums are more popular here; sugar maples less so. The sugar maple in the southeast section of the park was removed in 2020 after declining for years. A victim of climate change?

Global warming is marked on this map by the change in color of the Philadelphia region from the light green zone 6 to the yellow zone 7 over just sixteen years.

One of two remaining sugar maples

Our sole sweetgum

Biology and History

Biology is often interwoven with historical events. For example, along the southwest corner of the park, there used to be multiple mulberry trees forming a nice broad-leafed pergola. Like the golden rain trees, the pagoda trees, and the kousa dogwoods, the mulberries are native to Asia. For 5,000 years in China, mulberry leaves were fed to caterpillars of the Bombyx genus on an industrial level. The caterpillars would spin cocoons of silk, which would be harvested by dropping the cocoons in boiling water before the moth emerges. The cocoons would then be unraveled, with each cocoon yielding a single thread up to one mile long. These threads would be combined to make yarn and then turned into cloth on a loom. The medieval western demand for this luxurious fabric inspired land treks to China along what would be known as the Silk Road. Later, Philadelphians James Logan, Benjamin Franklin, and Robert Morris imported mulberry trees in hopes of starting a silk industry here. One of the larger silk businesses in Philadelphia, the J. C. Graham Trimming factory, would eventually settle at the northwest corner of the future Baldwin Park, as discussed here. Textiles were the largest industry in Philadelphia at this time, even bigger than the metal trades that dominated our neighborhood. For more on the silk industry in Philadelphia, see outside article here.

Louis Pasteur, around the time of the Graham factory's beginnings in Philadelphia, was persuaded by the French government to solve a problem: a disease in silk worms devastating the silk industry there. He isolated the causative fungal microorganism and developed a method to destroy the moths spreading the disease before the contagion caused too much financial loss. This episode and similar events induced Pasteur to formulate one of the greatest advances in medicine, the germ theory of infectious disease.

World trade routes, a major medical breakthrough, Philadelphia's silk industry (including a local one): all from a little tree and caterpillar.

Mulberry trees along the fence on the southwest section of the park in August of 2021. Mulberry trees are dioecious, with separate male and female trees, and almost all of these trees are male and non-fruit-bearing. There are at least two females among them, best seen from 19th Street. These fast-growing mulberries were pushing into and through the fence along the south side of the park. In order to protect the fence, they were cut down, except for the large mulberry along the wall. All that is left of them are their stumps.

Native vs. exotic vs. invasive

The mulberry, Morus alba, is an example of an invasive non-native tree. Have you ever heard the terms "native," "exotic," or "invasive" plant? A plant native to the United States has been here for hundreds of years, and most definitions state that native plants were here before the Europeans first came to the land that was to be the United States. Any plant brought after that time is a non-native or introduced species. The term exotic is similar, but means a plant that has recently been added to a continent from another continent. Many of these non-native plants were brought here accidentally as seed mixed in with cargo, but many were brought intentionally by humans, for humans. The mulberry is a non-native plant brought to this country for use in the silk industry.

Also on the other side of the fence, along the north lip of the wall of the Callowhill Cut, are two tall paulownia trees (Paulownia tomentosa, Royal Empress Tree), and there is another one on the east side of the 19th Street bridge next to the Park. There are also many paulownias in Logan Circle circumferentially arranged around the fountain. Alongside the two paulownias beyond our fence are a large mulberry, a catalpa (Catalpa speciosa, with the foot-long rigid seed pods), and a Siberian elm (Ulmus pumila) with one section fractured from the main trunk, and a callery pear. All of these are considered invasive species in the northeast United States, and all except the catalpa are from Asia. Using the paulownia as an example, it produces beautiful blue flowers before leafing out with very large leaves in the spring, and was brought to the United States for use as an ornamental. It will grow in almost any soil, produces millions of wind-scattered seeds, and is fast-growing. These same characters have allowed it to spread to the detriment of native ecosystems and it is considered an invasive species. It crowds out other species with its rapid seed germination and shades them out with its fast growth and huge leaves. Connecticut has banned the sale of paulownias for this reason. One reason paulownias are prominent along railroad tracks (and former railroad tracks like in the Callowhill Cut) is that the soft seed pods were used as "packing peanuts" around Chinese porcelain before the use of styrofoam. These seed pods would rupture and release their seeds during rail transport.

The catalpa is a tree native to the warmer southern region of the United States. There was a craze of planting catalpas as ornamentals in the 19th century, and apropos to Baldwin Park, the catalpa was planted extensively along railroad tracks as timber for railroad ties. In the northeast it is outside its native range, and acts similarly to an invasive of more distant origin. See outside article here for more on this.

The callery pear is a popular street tree in Philadelphia. The flowers have a distinct odor that some people find unpleasant. The fruits are too tiny for human consumption, but birds will eat the fruit and spread the seeds widely in their droppings.

Six invasive trees hanging over the wall on the north side of the Callowhill Cut.
From right to left (west to east) are a paulownia, a mulberry, a catalpa (with the hanging straight seed pods), another paulownia, a Siberian elm, and a small callery pear on the eastern end. All of these are also invading the wall with their roots and distorting the cast iron fence with their trunks.

Evolutionary Psychology

Evolutionary psychology seeks to identify which human psychological traits are adaptations shaped by natural selection. Is our love of park settings one such trait? Arboreal primates evolved about 60 million years ago. Human ancestors and chimpanzees took separate paths about 7 million years ago. Evolutionary aesthetics posits that our extinct human ancestors found comfort in being in landscapes that contain green meadows, a copse of trees, well-worn paths, and water features. The trees were especially appealing if they were low-branching trees that made upwards escape possible. Today, our subjective preference for such settings in landscape art and architecture is a remnant of the Pleistocene aesthetics that had survival advantages. Baldwin Park has lawn, groupings of trees, and delineated paths. When initially designed by Athena Tacha in the 1980s, the park was to have had a waterfall as well, but this feature was nixed by the City due to maintenance issues. For more on evolutionary aesthetics, see Wikipedia article here.

The cedar of Lebanon in the west end of the park is a beautiful tree, maybe more appealing because of its easily climbed branches that allow rapid human escape from predators.

A trick question for students: if a branch off the trunk of a tree is now three feet off the ground, how far off the ground will it be when the tree is three times taller? The answer is three feet. Trees grow at their apical meristems (the tips of branches and roots) and woody trees also grow circumferentially in girth. There is no meristem near those low cedar of Lebanon branches where they attach to the trunk. They stay at the same height unless they die from being shaded out by the higher branches, as happens with the white pine branches.

Worms

Charles Darwin's last manuscript, published in 1881 one year before his death, was titled The Formation of Vegetable Mould Through the Action of Worms, with Observations on their Habits. On his Sandwalk trips, he watched the earthworms in the loam and connected their actions to the slow sinking into the earth of the giant monolith's at Stonehenge. The worms basically eat and defecate soil, extracting the nutrients as they go, while displacing the digested earth to the surface. Darwin set up a circular stone and ruler in his garden to watch this process locally and on a small scale, but then extended, as he had with natural selection over geologic time, the local worm action over a brief span of time to larger consequences. Despite the specificity of the book's title, the book was a best-seller in garden-crazy England.

Worm castings at the surface on the lower bed of the center landscaping.
Given enough time and enough worms, those central plinths will one day be underground.

Artificial Selection and Man's Best Friend

Darwin goes into much detail about pigeon breeding in the first chapter of On the Origin of Species. He was building his case for the mutability of species over just a few centuries under the control of human breeders. His goal was the extension of such thinking to the mutability of all species, over millions of years, under the control of natural selection acting on random variation. Later in the book he also says the following about dogs:

"if... it could be shown that the greyhound, bloodhound, terrier, spaniel and bull-dog, which we all know propagate their kind truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many closely allied natural species..."

Darwin could not believe that the diversity of dogs could be derived from just one species, and believed that there were at least three origins of dogs. We now know that all of those French bulldogs, Maltese, and goldendoodles that make up the predominant visible fauna in the park are derived from just one species, the gray wolf. This is analogous to Brassica oleracea, which is one plant species, but has been shaped by humans (artificial selection) into cabbage, cauliflower, kale, kohlrabi, Brussel sprouts, and broccoli.

Artificial selection by humans has shaped the gray wolf into this cute short-faced, short legged wolf who could never make it in the wild. In fact, they almost cannot make it under domestication without human help from the very beginning of their lives. Most bulldogs are birthed by Cesarean section due to being bred for large heads that cannot pass through the birth canal.

Also in the first chapter of On the Origin of Species Darwin discusses Ancon sheep. In 1791 a lamb was born in Massachusetts with a normal head and torso, but short legs. The short legs made confining the sheep in pasture easier, requiring shorter walls. Through selective breeding, this type of sheep became a fad but due to other health issues seemingly unconnected with the short legs, the breed died out in 1876. Darwin used this breed as an exception to his principle of physical change in bodies due to accumulated variations acted on by natural selection. He called that first short-legged lamb a "sport," meaning a sudden and significant heritable variation.

These sheep today would be called achondroplastic, a type of dwarfism characterized by short legs with otherwise normal size head and torso. Humans, including some famous actors, can have this same autosomal dominant trait. Several breeds of dogs are also achondroplastic, bred for pursuing tunneling animals (dachshunds) or herding livestock (corgis). Mostly today, they are just selected for appearing cute. Just as in the sheep, however, the human preference for the short legs predisposes these dogs to other skeletal risks like herniated discs.