Darwin’s ‘Origin’ (ch.11-12): The geography of nature

Every day this week I will be publishing excerpts from my distilled version of On the Origin of Species. If you never have, or never will read it, hopefully this will give you a sense of what an amazing book it is. 

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Chapter 11: On Geographical Distribution I

Very few species have perfectly continuous ranges, and some even exist as scattered fragments of populations spread far and wide across the globe. How can these scattered forms have originated independently in disparate locations? Special creation seems to answer this question best. Yet, a closer look at species’ distributions show that there is no need for miracles.

The geographical distributions of species seem little related to environment or physical conditions. The conditions in North America and Europe are very similar, for example, yet these regions share almost no similar inhabitants. Conversely, over the length of South America, physical conditions vary dramatically. This does not seem to worry most of the inhabitants that range freely through the entire continent.

Isthmus of Panama map

It is very evident that natural barriers such as mountain ranges and large deserts determine the boundaries of different faunas and floras far more than physical conditions do. For example, the marine creatures flanking either side of the narrow isthmus of Panama differ far more from each other than do many species separated from each other by thousands of miles of continuous ocean.

Species inhabiting continuous tracts of land or sea always share some similar characteristics, like the beaver and musk rat in North America, that perform similar functions to the coypu and capybara in South America. These similarities can only be explained by a unique origin for each species.

There are some difficulties to my theory of course. Some areas, separated by seemingly insuperable barriers, do share species, such as the strangely uniform fauna and flora found on even mountain peaks separated by thousands of miles. For example, the alpine species of North America are almost identical to those in Europe, and both resemble many Arctic species.

The Glacial period, etched so deeply on the northern landscape offers us an explanation to this mystery. The vast ice-sheets that blanketed the northern hemisphere during this period allowed alpine species to range southwards and to displace the temperate inhabitants, spreading freely over the vast solid mass that bridged the current Atlantic Ocean.

When the ice-sheets retreated, the Arctic species retreated with them, northwards and to higher altitudes, followed closely by newly dominant temperate species. This hasty retreat left stranded populations of Arctic-dwellers isolated on mountain tops, bearing a strong similarity to those on other mountain peaks separated by thousands of miles of inhospitable warm lowlands. Even for apparently miraculous examples of multiple isolated forms, special creation is simply not necessary.


‘a mountain is an island on the land; and the intertropical mountains before the glacial period must have been completely isolated; and I believe that the productions of these islands on the land yielded to those produced within the larger areas of the north just in the same way as the productions of real islands have everywhere lately yielded to continental forms, naturalized by man’s agency.’ 


Chapter 12: Geographical Distribution II

Aside from the distribution of alpine species, there are two more that demonstrate the fallacy of the special creation argument, but are explicable through my theory of migration and modification.

Freshwater species appear confined to specific and discrete habitats, constrained to rivers, lakes and waterways. Yet, I remember my surprise on finding that the freshwater creatures I was collecting in Brazil were peculiarly similar to those I had seen in Britain. In fact, these creatures seem to range through water bodies across vast areas, even though inhospitable dry land divides their potential habitats.

Galapagos island engraving

This ranging might be partly due to the adaptations many have that allow them to cover some distance overland at particular life stages. It might also be due to accidental transport- molluscs clinging to weed fragments on duck’s backs, seed-charged mud trapped in the feet of water birds, or even fish picked up in windstorms. I have experimented by suspending a ducks foot to which clung tightly many newly-hatched molluscs. They survived nearly 20 hours in the air, which would be quite long enough for a bird to fly hundreds of miles.

The unusual inhabitants of oceanic islands are the final and most important case I will discuss. They have certain surprising characteristics, including the relatively low numbers of island species compared to continental areas, counterbalanced by the strikingly high numbers of species found nowhere else. Islands seem to lack terrestrial mammals, yet have plenty of bat species. They almost ubiquitously lack amphibians, and herbaceous plants that are dwarfed by trees on the mainland are often tree-like themselves on islands.

Most of these island anomalies can be accounted for through my theory. Islands have few species because the seas between them and the mainland, sometimes thousands of miles, prove a generally impassable barrier to most organisms. Few reach islands to colonise them. This same isolation, though, provides new opportunities. New organic relationships can be formed within the unique societies and physical conditions on islands. Immigrants may become modified over time as a result, and come to differ from their mainland ancestors, while retaining similarities. Organisms can often take up entirely new roles on islands, such as the giant flightless birds of new Zealand or the large reptiles of the Galapagos Archipelago taking up the role of absent land mammals.

Moa kiwi photograph

The factors affecting geographical patterns of forms can be seen within archipelagos, such as the Galapagos. Each island is inhabited by a unique species of land birds, each shaped minutely to the environment on its island. Yet, all the birds are closely related and share many similarities. This suggests that a single mainland species migrated over long ago, spreading from island to island, modified as it went. Crossings were probably rare due to the deep seas and low winds between islands, so the birds of different islands would diverge without interbreeding.


‘Nature, like a careful gardener, thus takes her seeds from a bed of a particular nature, and drops them in another equally well fitted for them.’


Previous:

Darwin’s Origin: Introduction

Darwin’s Origin, chapters 1-3: variation and struggle

Darwin’s Origin, chapters 4-6: natural selection and some difficulties

Darwin’s Origin, chapters 7-8: instinct and hybridism

Darwin’s Origin, chapters 9-10: the geological record

Next:

Darwin’s Origin, chapter 13 and conclusion

Brechth rose

© Natalie Lawrence: The Manticore 2018
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