• economists believed that resources were limited and population would grow to use up any increase in the supply of jobs and food, so there could never be increases in the living standard of the population as a whole
• the supply of land is limited
  
• because the majority will always earn subsistence wages, the market for comforts and luxuries will only be the small number of rich.  Therefore such products can be made by specialized craftspeople, there will be no demand for mass production
• read the first full paragraph on page 13 carefully

What are the limits on an organic economy?

• classical answer: land
• Wrigley's answer: energy
  

A similar argument for energy

• if production is based on human and animal muscle, the amount of energy provided by the crops you can grow is limited
• if you had to depend on the growth of wood for fuel you couldn't produce much metal
• an organic economy (one dependent on human and animal power and plants for fuel) cannot support an industrial revolution

Incomes tend to return to the subsistence minimum (enough only for minimum food, shelter, clothing and fuel), therefore little demand for comforts and luxuries
the percentage of income an average family spends on food approximates the percentage of workers in agriculture (because if families spend 75% of their income on food then there isn't a market for workers to make other things)

Wrigley's key argument:
it wouldn't be possible to have sustained economic growth without dramatically increased use of inorganic sources of energy

• now energy is no longer a limit, and coal is also a much denser source of energy
• but you also need a decline in population growth
• when coal and oil become a major source this is actually a transition from a sustainable to an unsustainable economy.  Some day, centuries later, we will run out of coal and oil