‘Some difficult problems’ describes some examples of biological phenomena that are difficult to explain including the evolution of flight, eyes, and protein molecules; and why ageing occurs and how it affects reproduction and death rates. Critics of evolutionary theory by natural selection often ask how complex adaptations evolve. In the evolution of organisms and their complex machinery, many aspects are modified (adapted) versions of pre-existing structures. A succession of small changes to a structure that already works, but can be improved, can therefore produce large evolutionary changes. Two other major and largely unsolved problems in evolution are the origin of the basic features of living cells and the origin of human consciousness.