Permaculture contains ethics that lead to principles. Some of the ethics overlap the principles. The principles lead to methods. The principles and the methods overlap each-other. The methods lead to outcomes. The outcomes overlap everything because it works like the our cells work in our body. It is a self regulating perpetual cycle, and we are part of it. There is no "one solution." To me, permaculture is the synergy of all elements. Capt Benny Pants, an online participant in the Permaculture Design Course 2008.
The Aim of Permaculture
Permaculture is a reaction to the developments of "military adventurism, the bomb, ruthless land exploitation, arrogance of pollution, and general insensitivity to human and environmental needs". In other words, it seeks to solve the many problems of the modern world, from public health and education to energy security/independence and climate change, which in the eyes of some have have their roots in food production  by finding a solution to an inefficient, unbalanced, and consequently unsustainable and destructive system. Aside from serving as a replacement culture and attempting to save humanity, and all life on earth, by being ethical permaculture has more specific, small scale goals on how this can be accomplished:
- Care for surviving natural assemblies
- Rehabilitate degraded or eroded land
- Create your own complex living environment
- Earthcare – recognising that the Earth is the source of all life (and is possibly itself a living entity- see Gaia theory) and that we recognise and respect that the Earth is our valuable home and we are a part of the Earth, not apart from it.
- For example:
- Conservation of Biodiversity
- Clean air and water
- Restoration and conservation of forests, habitats and soils
- Recycling and pollution reduction
- Conservation of energy and natural resources
- Appropriate technology
- Peoplecare – supporting and helping each other to change to ways of living that are not harming ourselves or the planet, and to develop healthy societies.
- For example
- Health and well-being
- Nourishment with good food
- Lifelong learning
- Right livelihood and meaningful work
- Community belonging
- Open Communication
- Trust and Respect
- Fairshare (or placing limits on consumption) - ensuring that the Earth's limited resources are utilised in ways that are equitable and wise. Also if a wall retains heat, pass on the heat to a plant that needs it; if a plant provides shelter, place it on a wall that needs cooling.
- For example
- Networking and sharing
- Distribution of resources and wealth
- Reduction of consumerism
- Rethinking current notions of growth, progress and development
- Making a contribution
As permaculture's goal is creating a sustainable ecosystem, it is useful to understand some basic principles of systems, and natural systems, as well as how these translate into creative design principles.
Bill Mollison in "Permaculture: A Designers' Manual" posits that the most fundamental permaculture principle is:
- The Law of Return
It is a recognition of the thermodynamic laws of energy: No energy can be created or destroyed, it can only be transferred, but never with perfect efficiency; thus entropy in systems is always on the rise. However, this view of entropy only applies to closed thermodynamic systems, whereas an ecosystem is an open thermodynamic system, and as the second rule for open system is that the increase in the internal energy of a system is equal to the amount of energy added to the system by matter flowing in and by heating, minus the amount lost by matter flowing out and in the form of work done by the system, making sure that input does not fall below the outflow. Thus the crux of this becomes two fold:
- To take full advantage of energy before it is let out of their system.
- To always put back what you take out.
The keyword becomes: Exchange
The driving factor in balance in a natural system is:
- The Principle of Disorder: Resources that surpass the ability of a system to absorb them are pollutants and resources that fall short of a system's needs are deficiencies; both deficiencies and needs push the system towards disorder.
To understand this, first we need to look at the elements of natural systems. These are resources and pollutants. Resources are not a limited category. Though most fundamentally the only resource is energy: mechanical, thermal, chemical; when we dive into the realm of biology, resources bloom into soils, air, water, sunlight, seeds, plants, forests... So, we find it helpful to look at resources by category. The following are characteristics of different resource categories:
- Resources that increase with use.
- Resources that are unaffected by use.
- Resources that are available only for a short period of time.
- Resources that are reduced by use.
(Adapted from Permaculture: A Designer's Guide)
It should be recognized that one to three are resources that are readily produced in natural systems and are a solid and sustainable base, whereas four is a dead end road, with unusual byproducts, and it's use should be carefully considered and used only to benefit the ecosystem.
And what of pollutants? In fact, resources themselves are pollutants (as pollutants are resources).
What separates a pollutant from a resources is merely a matter of Balance
There are many definitions on yield, but most are a variation of giving, based on some effort or investment.
What is Yield?
However, the concept of yield in permaculture rests on a fundamental principle (which is true at least until humans invent some sort of non-dependent abiological being, which... well...):
- The Role of Life in Yield:"Living things are the only effective intervening systems to capture resources on this planet, and to produce a yield."
The importance of this element is apparent in the aims of permaculture. Even technology as we know it rests on biological systems to support it, as it rests on humans for support and we cannot live without a biological system to support ourselves so, to create our own complex living environment we need a biological system, and to rehabilitate degraded or eroded land we should produce yields which support biological growth, and to care for surviving natural systems we need to have sustainable yield. Again:
- Yield requires a biological system.
- Yield should support biological growth.
- Yield should be sustainable.
|Learning Excercise: Can you create a definition for yield that is compatible with these principles?|
Practically this means that it is important that the system yields some resources that you can use, or to see it another way, that you figure out how to use some of the resources that the system yields.
While many definition for yields may exist, the definition that satisfies the restrictions of a permaculture world-view is:
- Definition of Yield:"The sum total of surplus energy produced by, stored, conserved, reused, or converted [within the system, where surplus energy is that which exists beyond the needs of a system for its growth, reproduction and maintenance.]"
Types of Yield
Still, there are many types of yield:
- Inherent Yields (available in the system before design) and
- Designed Yields (made available by design).
- Energy Yields are the base form of all yields but yields may take the form of
- Product Yields are a possible form of energy yields.
- Undelineable Yields and the third category of yields which, while ultimately based on products or energy, are more a little more illusive. As our observational skills become more detailed these yields may become more delineable, but as of yet, and forever after, there will be some yields which we do not fully understand.
- Short Term Yields and
- Long Term Yields are simply yields in time and is more a recognition that different yields may occur on different time scales, rather than a fixed time scale.
- Cyclical Yields
- Export Yields
- Imported Resources
Cyclical yields are yields that do not leave the system, but are created within and used within the system. Examples are grass, turned milk, turned human waste, turned manure and cooking fuel. You might notice that cooking fuel is inevitably released into the atmosphere and may or may not be absorbed by the carbon storing trees of your plot of land. Thus, it might be an export yield, which leaves the system. Other examples of yields which leave the system are meats and vegetables sold or traded, or manure or seeds sold and traded. The question then becomes, is this outward flow sustainable? What are the import resources (or import yields) that you acquire? Some systems may be in need of some supplements which can take the form of organic 'waste' (aka compost material), seeds, bacteria (for seed planting), or perhaps even in the form of information. These import resources/yields may be yields of parts of the system are used to attain them (profit from products used to buy seed, trade of resources, etc.), or only resources if you're lucky enough to be getting them for free (like municipal mulch; though, really, you should realize,in the larger system you ARE taking them from somewhere.)
It is important to realize that the boundaries of a system are flexible and dependent on your perspective. For instance, back to the grass example, a location may have grazing land, a vegetable patch, and a house. The grazing land may produce milk and the vegetable patch vegetable which are consumed by humans who inhabit the house, but produce waste which is then converted, by bacteria and worms into viable manure for the vegetable patch. Here, on a local level the milk is an export yield for the grazing land (unless fed to calves), while it remains a cyclical yield for the greater system, but serves as an import yield for the vegetable garden (whose vegetables are cyclical unless sold) until it is released as cooking fuel, an export yield (unless the carbon is re-captured by carbon storing forests). These finer delineations may be useful in troubleshooting and assessing sustainability of a system and its subsystems.
Factors in Yield
There are many factors that increase and decrease yield in a system. The following section is heavily adapted from Bill Mollison's, Permaculture: A Designers' Guide
- Limits to Yield: A system's yield is not fixed, rather it is a function of the ability of that system to efficiently use the plethora of resources in a system.
These are situations or techniques that can be used to create and improve yield.
- Niche use of resources or space
- Soil Reconditioning: increases root penetration and water infiltration/absorption (decreases runoff), and supplies essential nutrients.
- Earth-working: Assures even, low work irrigation (no water-logging, or dry areas), reduces soil loss from runoff or salting, may reduce need for improve efficiency of pumps, makes water use-techniques possible.
- Water storage, recycling and diversion: harnesses superior efficiency and productivity of water systems and their animals compared to their land-based counterparts, as well as improved irrigation potential, nutrient quality from aquatic wildlife manure, as well as the effects of aquatic wildlife on pest and weed control, and additional benefits from micro-climatic buffering.
- Application of Reforestation, Wild Life Corridors and Windbreak/Food-forest: Shelters (increasing plant and animal yields, as well as micro-climate buffering both above and below ground), increases carrying capacity through shrub and tree foraging, recycles nutrients via legumes and trees, provides forest products (nectar, seeds, firewood, etc.), serves as general wildlife corridor providing habitat for pest predators, increased precipitation (due to night condensation, water penetration and , trees cross-wind), allows for improved perennial crops, reduces cost and increases capacity by providing self-forage browse (in the form of drought-proof stock-feed and medicinal plants), provides durable timber (for construction material), reduces livestock carcass loss due to elements (sweating and shivering, as well as reduced grazing due to seeking shelter), increased crop production due to sheltering, and reduced water loss from nearby water bodies.
- Integration of structures and landscape: can provide improved energy saving, conservation and production
- Re-routing materials or energy
- Using effective shapes
- Zone, sector, slope, orientation and site strategies
- Extending Yields: includes use of inclusive harvesting of leaf, fruit, seed and roots; diversity in the system; sequential stacking like the use of perennials, early/mid/late and long vs. short season varieties and planting patterns; use of self storing species such as tubers, hard seeds, fuel wood, and rhizomes, preservation techniques; and regional trade between communities in order to spread and preserve yields to provide fiscal and logistic benefits.
- Yield Storage
- Increasing cyclic frequency
- Tessellation of cycles and successions
- Low maintenance elements
- Proper supply of key nutrients
- Plant and animal guilds
- Application of technology/problem solving tools/ideas: electrics fencing improving grazing accessibility and rotation potential.
- Energy efficient structures.
- Efficient routing of resources
- Proper storage
- Efficient work (no tillage)
- Durability, care and maintenance
- Catching run-off resources
- On-Premise Fuels
- Low or no-tillage farming: preserves soil and water resources, and reduces energy expenditure and time between crops.
- Social Development: overcoming cultural barriers to efficient use, expanding choices in culture, removal of socio-legal barriers, and positive action to effect socio-economic and legal change.
- Innovative use of resources
- Cooperative endeavors, pooling and sharing: labor exchange, produce and marketing cooperatives, CSA, and structures to aid resource management.
- Financial recycling: may include local credit unions,
- Socio-legal-economic support for all energy inputs and outputs
- Market Based Crop Selection
- Diversified Marketing: Self-pick, mail order, direct dispatch,road-side sale.
- Processing of Products: growing, developing, and even manual processing of resources to higher order or more refined products.
- Income from Providing Services: income from field days and educational courses, rental or income from urban visitors,
- Direct investment from urban sources.
- Harmonious connections
- Making informed choices
It would be superfluous to list all the ways you could NOT use the above to improve yields, here are a few examples of inefficient systems applied today. In fact, there are many pervasive modern designs that are inefficient.
- Systems like paved roads, or concrete structures halt production of many ecological resources, and are inefficient conservers.
- Many systems which depend on non-renewable, non-recyclable resources, such as fossil fuels.
- Systems which require undue upkeep, for instance the superhighways and sewers of large cities and states which exorbitant funds on upkeep.
- Many man-made systems also depend on destructive/pollutive systems.
- Counter-yield waste management legislation and civic projects.
- Cultural limitations such as a predisposal against particular plants and animals which can lead to negative excesses of others.
- Search for "Maximum Product Yields" by employing genetic selection, increased fertilizer and water, decreased competition for resources; these factors also contribute to imbalance due to unusual factors in such high yield plants and animals, such as limited reproductive ability, and low stress tolerance.
- Limited biodiversity expose systems to risks in the case of changes in climate or environment that could adversely affect yields due to crop or livestock losses, as well as showing inverse correlation to the resources available for yield in some systems.
- Old age of elements in systems usually correlates to maturity, which means that energy is spend on maintenance, rather than growth, reducing yield. A range of ages in a system can ensure degrees of stability as well.
First and foremost is mis-use and disposal of resources that results in pollution: An ideal permaculture system does not use resources which permanently reduce yields of sustainable resources. This means that if use of a resource/yield results in a "waste" that cannot be absorbed by the system (pollution), and thus creates chaos in the system, it should not be used (or a way should be found to absorb the resource.
- The Principle of Cyclic Opportunity: Every Cyclic event increases the opportunity for yield. To increase cycling is to increase yield.
A cycle repeats. You take an input, use it, output, and re-use. This is done everywhere in the universe, on a range of time scales from nano-seconds to millennia and perpetrated by living and non-living things alike. Cycles can be entropic, or in the case of living, open systems, increasing in energy. Some cycles have as output something that cannot be re-used by itself, this is where a niche in a cycle can appear: one or more elements can bridge the gap in a continuous garbage out-garbage in cycle in which there is no waste. In nature, each new element in the cycle offers a new opportunity for efficient conservation of the open system's input (for nature this can be any number of things, but sunlight takes a special place). Some things to remember:
- Life itself is not the end but simply the facilitator of the many stepping stones.
- Niches in a cycle are always filled by something.
- No element in a cycle claims full right to the system.
- Conflict is generally not due to space alone, but being out of schedule within a cycle.
- Stacking uses cycles and can be applied to plant and animal life, both wild and domesticated (grazing).
The basic trophic pyramid is something to the effect of plants eaten by bugs, eaten by other animals, and still other animals, eaten by humans.However, nature is much more complex and food webs attempt to make this complexity apparent. Feedback and the cyclical nature of ecological systems is also an important aspect to consider. In addition to consuming, plants and animals also contribute to the food sources of lower animals. Further, mature specimens have different food intake a requirements and uses than growing specimens, which is a factor to be considered in cropping and system growth. As a large part of very important vegetation is not digestible by all species (most pertinently humans), the food chain provides processing for indigestible foods for larger species. Sharks cannot live on plankton, nor humans on grass and tree-leaves.
When considering your place in the food system it's important to consider the real socio-ecological effects of the complete system on which your diet depends, including ecological destruction like deforestation and biodiversity dives due to mono-cropping, as well as social factors such as patented crops (soya), as well as the sustainability, efficiency, and effectiveness of the products your buying from farms.The whole system should be considered, both taking and giving, with special consideration given to urban settings and the transport, storage and waste pathways involved.
When complete systems are evaluated considering food sources, and special availability factors (sea sources, extreme climates), vegetarian, omnivorous and even primarily carnivorous diets all prove themselves as extremely valid provided they match their position in the system.
Complexity and Connections
Efficiency is increased with the number of cycles that occur in a system, and stability by the number of energy pathways, or connections available in the system. Consider the species found on an average plot of corn or soy in an industrialized agricultural system which has been leveled, drained, and fertilized for a specific crop, and compare it to that found in places with varied micro-elevation, drainage, and energy pathways, such as hill and valley systems; compared even to the complete biodiversity of plant and animal systems of a nearly mono-specie coastal mangrove system in which mobile species occupy a great variety of niches within a single tree or swamp within the high/low tide schedules, such simple systems are frightfully limited. Other simple systems can be found in newly desertification areas, which might harbor as few as 150 woody plant species, compared to the 3000 woody plants of old desert ecosystems. Constructive changes can be made to such ecosystems by introduction of suitable species to such ecosystems and fostering long term re-afforestation, rather than soil loss and environmental degradation due to cultivation of other mono-crop systems.
The complexity of a system increase the number of niche cycles that occur. The complexity of interactions in such systems increases as a square of the number of elements as each element interacts with the rest. While it is of course possible that two elements in a system may not have a direct interaction, the ways in a which an earthworm or bird may interact with other elements in the system are a mystery even unto themselves. The lesson to take is the value of connection between elements, but realize that these connections do not confine themselves to our perceptions, but follow instead their own nature and are able to extend beyond our design expectations creating new relationships with elements we had not considered.
While complexity can be a positive factor supporting stability, it can also be a driving force toward instability as cooperative complexity is replaced with competitive or inharmonious complexity. It's a matter of what you need out of a system. 
Order and Chaos
The true test of order vs chaos is that of yield. Order is not the visual simplicity, squares and ovals of a system, but the mutual sustainability of supporting connections within that system. As a system devolves into entropy and more and more energy becomes unusable, chaos is found as the system struggles to fulfill the needs of it's existing elements and new elements move in to replace the ineffective system.
- Principle of Disorder: Order and harmony produce energy for other uses. Disorder consumes energy without yield. Neatness, tidiness, and uniformity are the mark of energy-maintained disorder.
Order is balance and productive cyclical flux, and chaos is the state of non-cyclical change which devolves into system energy loss, but what you look for in yields and elements defines your perception of systemic order or chaos. To the soya farmer, anything other than soy in their field is a loss, unless they can bring themselves to accept alternative yields, or see how these elements work for them.
Permitted and Forced Functions
While all elements in a system interact and provide for the system, forcing these interactions can lead to collapse. Just like people, who appreciate a degree of variety and depend on their rest, so animals do to. Elements can be utilized, but not stressed, through proper design. The Principle of Stress and Harmony:
- Stress is the prevention of natural function, or forced function.
- Harmony is the integration of chosen and natural functions, and the supply of essential needs.
Diversity, and biodiversity, when not integrated into the system, does not necessarily contribute to stability or increased yield. In fact, maintained systems can have many elements which are not integrated with the system and if left unattended will disappear. Some diversity is desired for yields, this is where we come in and manage and maintain diversity. Diversity, if not integrated, can create disorder and chaos. A balance must be found. In some cases, diversity can even be incompatible, as in apple and walnut trees, but can be corrected, as with the mulberry. 
- Principle of Stability: It is not the number of elements, but the beneficial connections between them, that leads to diversity.
Therefore, adding diversity, just for diversity's sake is not necessarily positive. However, when made to do something useful, it can be a benefit. Here is where information, which is a resources only when acted upon, becomes useful.
The distinctions between richness(the number of species in an area), diversity(their relative abundance), and evenness(how species contribute to total biomass) are important distinctions to consider.
Different stressors lead to different situations, such as the difference between pruned and uncut fields; diversity of plants vs animals may shift.
Stability in ecosystems is not about the end point, but about the regulation over the lifespan of a system. Unlike the rigid stability of a concrete pylon, stability here is more like that of a slackliner. Aside from obvious imbalances, instability can be caused by slow nutrient loss due to fire and water (even simply water leaching to lower depths) even in old, stable ecosystems unless there are new nutrients made available somehow. Pathogens can also cause instability as the environment adjust to new pressures. Humans can play a role in maintaining stability, but short and long term.
To see how these principles play out when designing a sustainable system that both gives and receives effectively are:
- Work with nature, not against it: This involves using the positive elements of a system to support more positive elements, rather than only seeing their negatives. In essence,
- The problem is the solution: Having a flexible pattern of application is the key to this design principle. If you can do this then it allows you to
- Make the lest change for the greatest possible effect: This involves choosing the most suitable location, plant, animal, etc. for a given system, or better yet, using what you have. And remember,
- A system's yield is theoretically infinite: the limit is not what the system gives, but what you have the creativity and prowess to use, whether it be vacant niches, or a new use or market for something. Finally,
- Everything gardens: Every element, from bacteria to fungi, plants to animals, affects its environment. 
Problem Solving Methods
- Improving Tools
- Collecting Extensive Observations
- New Perspectives (Like a Eureka! - Perhaps a result of collecting observations)
- Tests and Trials
- Guesses (after all, you can't know everything. Educated guessing is best though.)
- Noting Unique Observations
- Accidents (accepting the unexpected in trial and observation)
- Patterning (deriving the effects from a series of events)
- Commonsense AKA. "management"
It will be useful to give a quick overview of the elements considered in premaculture design, both as a quick explanation for those who desire it, and as an introduction to prepare for later in the text where these factors will be discussed in more detail.
To understand balance we need to consider the elements of a natural system. First we need to understand how natural systems function. Bill Mollison suggests something similar to the following principles of natural systems, which are claimed to have come from Charles Birch (but reference cannot be found), and have been further modified here:
- No organism or material stays in one form forever.
- Materials and organisms require balance to continue existing.
- Disorder is caused by both too great, or too low a resource input.
- Maintaining global bio-chemical cycles is necessary to retain such a balance.
- While many limiting factors may exist, usually an imbalance is (initially) only of a few factors.
- Ability to change increases faster than the ability to predict effects of those changes. (Perhaps we need time to learn what the effects of these new forms of change are. For instance, global warming, which has been the result of unprecedented increases in fossil fuel use was not predicted until long after the ability for large scale had been put to use.)
- Living organisms are themselves ends, as well as serving as the means of other organisms to survive.
The following are restatements of principles of permaculture that appear in David Holmgren's Permaculture: [Principles and Pathways Beyond Sustainability http://www.holmgren.com.au]; Also see permacultureprinciples.com; For this exercise you will try to see which of the above principles apply to each of the following and how. In fact, though restated differently, are only another way of saying the same things. Can you see how? Can you make your own restatements?
- Observe and interact - By taking the time to engage with nature we can design solutions that suit our particular situation.
- Catch and store energy - By developing systems that collect resources when they are abundant, we can use them in times of need.
- Obtain a yield - Ensure that you are getting truly useful rewards as part of the work that you are doing.
- Apply self-regulation and accept feedback - We need to discourage inappropriate activity to ensure that systems can continue to function well.
- Use and value renewable resources and services - Make the best use of natures abundance to reduce our consumptive behaviour and dependence on non-renewable resources.
- Produce no waste - By valuing and making use of all the resources that are available to us, nothing goes to waste.
- Design from patterns to details - By stepping back, we can observe patterns in nature and society. These can form the backbone of our designs, with the details filled in as we go.
- Integrate rather than segregate - By putting the right things in the right place, relationships develop between those things and they work together to support each other.
- Use small and slow solutions - Small and slow systems are easier to maintain than big ones, making better use of local resources and produce more sustainable outcomes.
- Use and value diversity - Diversity reduces vulnerability to a variety of threats and takes advantage of the unique nature of the environment in which it resides.
- Use edges and value the marginal - The interface between things is where the most interesting events take place. These are often the most valuable, diverse and productive elements in the system.
- Creatively use and respond to change - We can have a positive impact on inevitable change by carefully observing, and then intervening at the right time.
Characteristics of a Permaculture System
These are all common characteristics that you will find in Permaculture systems. Each system will be unique as the conditions of climate, soils, aspects, culture, time & energy resources will guide its design & development
- Small scale land use. By using the least possible area, marginal land is preserved in, or returned to, natural ecosystems.
- Diversity of species & cultivars (types of plants) yields, microclimates, habitats, functions social roles & work.
- Interweaving of agriculture, animal care, forestry, aquaculture, (water food systems), wilderness management, foraging, economics, culture, & land form engineering (such as dams).
- Use of wild & domestic plants & animals. Rare, endangered & indigenous are included.
- Uses the natural ways of plants, animals & their relationships to the characteristics of landscape to create environmentally safe sustainable agriculture. This means energy & biological resources including water & soil, are conserved rebuilt, self-regulating & self- repairing.
- Long ~ term sustainability. Permaculture systems can be designed to adjust to environmental changes.
- Does not use resources with permanently reduce yields of sustainable resources.
To sum it up in a few words:
- Multi layered
- Conservative (of resources!)
- Low inputs
- High yields
- Mollison 1979, p.ix.
- Bill Moyer (host), Micheal Pollan (guest). (2008-11-28) (in en) (TV). Bill Moyer's Journal. US: PBS. Archived. Template:Citation error. http://www.pbs.org/moyers/journal/11282008/watch.html. ""the challenge is not just what we do with agriculture, it's connecting the dots between agriculture and public health, between agriculture and energy and climate change, agriculture and education. ""
- Mollison 1979, p. 13.
- Mollison, 1979, p. 18,
- Mollison 1979, p.16.
- ? (18 November 2010). "Dictionary.com;yield". http://dictionary.reference.com/help/terms.html. http://dictionary.reference.com/browse/yield. Retrieved 18 November 2010.
- Mollison 1979, p. 19.
- Mollison 1979, p. 18.
- Mollison 1979, p. 18.
- Stephen Battersby (15 November 2010). "Summon a 'demon' to turn information into energy". Newscientist. Reed Business Information Limited, Quadrant House, Sutton SM2 5AS, UK. http://www.newscientist.com/article/dn19723-summon-a-demon-to-turn-information-into-energy.html. Retrieved November 14, 2010.
- Mollison 1979, p. 19-20.
- Mollison 1979, p. 33.
- Mollision 1979, p. 33.
- Mollison 1979, p.17.
- Mollison 1979, p.23.
- Mollison 1979, p. 30-31.
- Mollison 1979, p. 31.
- Mollison 1979, pg. 32.
- Mollison 1979, p. 32.
- Mollison 1979, p. 33.
- Mollison 1979, p. 15-16.
- Mollison 1979, p. 12.
- Mollison 1979, p. 15.